[v3,1/4] KVM: Dynamic sized kvm memslots array

Zhiyi reported an infinite loop issue in VFIO use case.  The cause of that
was a separate discussion, however during that I found a regression of
dirty sync slowness when profiling.

Each KVMMemoryListerner maintains an array of kvm memslots.  Currently it's
statically allocated to be the max supported by the kernel.  However after
Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
the max supported memslots reported now grows to some number large enough
so that it may not be wise to always statically allocate with the max
reported.

What's worse, QEMU kvm code still walks all the allocated memslots entries
to do any form of lookups.  It can drastically slow down all memslot
operations because each of such loop can run over 32K times on the new
kernels.

Fix this issue by making the memslots to be allocated dynamically.

Here the initial size was set to 16 because it should cover the basic VM
usages, so that the hope is the majority VM use case may not even need to
grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
it'll consume 9 memslots), however not too large to waste memory.

There can also be even better way to address this, but so far this is the
simplest and should be already better even than before we grow the max
supported memslots.  For example, in the case of above issue when VFIO was
attached on a 32GB system, there are only ~10 memslots used.  So it could
be good enough as of now.

In the above VFIO context, measurement shows that the precopy dirty sync
shrinked from ~86ms to ~3ms after this patch applied.  It should also apply
to any KVM enabled VM even without VFIO.

NOTE: we don't have a FIXES tag for this patch because there's no real
commit that regressed this in QEMU. Such behavior existed for a long time,
but only start to be a problem when the kernel reports very large
nr_slots_max value.  However that's pretty common now (the kernel change
was merged in 2021) so we attached cc:stable because we'll want this change
to be backported to stable branches.

Cc: qemu-stable <qemu-stable@nongnu.org>
Reported-by: Zhiyi Guo <zhguo@redhat.com>
Tested-by: Zhiyi Guo <zhguo@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/sysemu/kvm_int.h |  1 +
 accel/kvm/kvm-all.c      | 99 ++++++++++++++++++++++++++++++++++------
 accel/kvm/trace-events   |  1 +
 3 files changed, 86 insertions(+), 15 deletions(-)

Peter Xu <peterx@redhat.com> writes:

> Zhiyi reported an infinite loop issue in VFIO use case.  The cause of that
> was a separate discussion, however during that I found a regression of
> dirty sync slowness when profiling.
>
> Each KVMMemoryListerner maintains an array of kvm memslots.  Currently it's
> statically allocated to be the max supported by the kernel.  However after
> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> the max supported memslots reported now grows to some number large enough
> so that it may not be wise to always statically allocate with the max
> reported.
>
> What's worse, QEMU kvm code still walks all the allocated memslots entries
> to do any form of lookups.  It can drastically slow down all memslot
> operations because each of such loop can run over 32K times on the new
> kernels.
>
> Fix this issue by making the memslots to be allocated dynamically.
>
> Here the initial size was set to 16 because it should cover the basic VM
> usages, so that the hope is the majority VM use case may not even need to
> grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
> it'll consume 9 memslots), however not too large to waste memory.
>
> There can also be even better way to address this, but so far this is the
> simplest and should be already better even than before we grow the max
> supported memslots.  For example, in the case of above issue when VFIO was
> attached on a 32GB system, there are only ~10 memslots used.  So it could
> be good enough as of now.
>
> In the above VFIO context, measurement shows that the precopy dirty sync
> shrinked from ~86ms to ~3ms after this patch applied.  It should also apply
> to any KVM enabled VM even without VFIO.
>
> NOTE: we don't have a FIXES tag for this patch because there's no real
> commit that regressed this in QEMU. Such behavior existed for a long time,
> but only start to be a problem when the kernel reports very large
> nr_slots_max value.  However that's pretty common now (the kernel change
> was merged in 2021) so we attached cc:stable because we'll want this change
> to be backported to stable branches.
>
> Cc: qemu-stable <qemu-stable@nongnu.org>
> Reported-by: Zhiyi Guo <zhguo@redhat.com>
> Tested-by: Zhiyi Guo <zhguo@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/sysemu/kvm_int.h |  1 +
>  accel/kvm/kvm-all.c      | 99 ++++++++++++++++++++++++++++++++++------
>  accel/kvm/trace-events   |  1 +
>  3 files changed, 86 insertions(+), 15 deletions(-)
>
> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
> index 1d8fb1473b..48e496b3d4 100644
> --- a/include/sysemu/kvm_int.h
> +++ b/include/sysemu/kvm_int.h
> @@ -46,6 +46,7 @@ typedef struct KVMMemoryListener {
>      MemoryListener listener;
>      KVMSlot *slots;
>      unsigned int nr_used_slots;
> +    unsigned int nr_slots_allocated;
>      int as_id;
>      QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_add;
>      QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_del;
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 75d11a07b2..c51a3f18db 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -69,6 +69,9 @@
>  #define KVM_GUESTDBG_BLOCKIRQ 0
>  #endif
>  
> +/* Default num of memslots to be allocated when VM starts */
> +#define  KVM_MEMSLOTS_NR_ALLOC_DEFAULT                      16
> +
>  struct KVMParkedVcpu {
>      unsigned long vcpu_id;
>      int kvm_fd;
> @@ -165,6 +168,57 @@ void kvm_resample_fd_notify(int gsi)
>      }
>  }
>  
> +/**
> + * kvm_slots_grow(): Grow the slots[] array in the KVMMemoryListener
> + *
> + * @kml: The KVMMemoryListener* to grow the slots[] array
> + * @nr_slots_new: The new size of slots[] array
> + *
> + * Returns: True if the array grows larger, false otherwise.
> + */
> +static bool kvm_slots_grow(KVMMemoryListener *kml, unsigned int nr_slots_new)
> +{
> +    unsigned int i, cur = kml->nr_slots_allocated;
> +    KVMSlot *slots;
> +
> +    if (nr_slots_new > kvm_state->nr_slots) {
> +        nr_slots_new = kvm_state->nr_slots;
> +    }
> +
> +    if (cur >= nr_slots_new) {
> +        /* Big enough, no need to grow, or we reached max */
> +        return false;
> +    }
> +
> +    if (cur == 0) {
> +        slots = g_new0(KVMSlot, nr_slots_new);
> +    } else {
> +        assert(kml->slots);
> +        slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
> +        /*
> +         * g_renew() doesn't initialize extended buffers, however kvm
> +         * memslots require fields to be zero-initialized. E.g. pointers,
> +         * memory_size field, etc.
> +         */
> +        memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
> +    }
> +
> +    for (i = cur; i < nr_slots_new; i++) {
> +        slots[i].slot = i;
> +    }
> +
> +    kml->slots = slots;
> +    kml->nr_slots_allocated = nr_slots_new;
> +    trace_kvm_slots_grow(cur, nr_slots_new);
> +
> +    return true;
> +}
> +
> +static bool kvm_slots_double(KVMMemoryListener *kml)
> +{
> +    return kvm_slots_grow(kml, kml->nr_slots_allocated * 2);
> +}
> +
>  unsigned int kvm_get_max_memslots(void)
>  {
>      KVMState *s = KVM_STATE(current_accel());
> @@ -193,15 +247,26 @@ unsigned int kvm_get_free_memslots(void)
>  /* Called with KVMMemoryListener.slots_lock held */
>  static KVMSlot *kvm_get_free_slot(KVMMemoryListener *kml)
>  {
> -    KVMState *s = kvm_state;
> +    unsigned int n;
>      int i;
>  
> -    for (i = 0; i < s->nr_slots; i++) {
> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>          if (kml->slots[i].memory_size == 0) {
>              return &kml->slots[i];
>          }
>      }
>  
> +    /*
> +     * If no free slots, try to grow first by doubling.  Cache the old size
> +     * here to avoid another round of search: if the grow succeeded, it
> +     * means slots[] now must have the existing "n" slots occupied,
> +     * followed by one or more free slots starting from slots[n].
> +     */
> +    n = kml->nr_slots_allocated;
> +    if (kvm_slots_double(kml)) {
> +        return &kml->slots[n];
> +    }
> +
>      return NULL;
>  }
>  
> @@ -222,10 +287,9 @@ static KVMSlot *kvm_lookup_matching_slot(KVMMemoryListener *kml,
>                                           hwaddr start_addr,
>                                           hwaddr size)
>  {
> -    KVMState *s = kvm_state;
>      int i;
>  
> -    for (i = 0; i < s->nr_slots; i++) {
> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>          KVMSlot *mem = &kml->slots[i];
>  
>          if (start_addr == mem->start_addr && size == mem->memory_size) {
> @@ -267,7 +331,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s, void *ram,
>      int i, ret = 0;
>  
>      kvm_slots_lock();
> -    for (i = 0; i < s->nr_slots; i++) {
> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>          KVMSlot *mem = &kml->slots[i];
>  
>          if (ram >= mem->ram && ram < mem->ram + mem->memory_size) {
> @@ -1071,7 +1135,7 @@ static int kvm_physical_log_clear(KVMMemoryListener *kml,
>  
>      kvm_slots_lock();
>  
> -    for (i = 0; i < s->nr_slots; i++) {
> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>          mem = &kml->slots[i];
>          /* Discard slots that are empty or do not overlap the section */
>          if (!mem->memory_size ||
> @@ -1719,12 +1783,8 @@ static void kvm_log_sync_global(MemoryListener *l, bool last_stage)
>      /* Flush all kernel dirty addresses into KVMSlot dirty bitmap */
>      kvm_dirty_ring_flush();
>  
> -    /*
> -     * TODO: make this faster when nr_slots is big while there are
> -     * only a few used slots (small VMs).
> -     */
>      kvm_slots_lock();
> -    for (i = 0; i < s->nr_slots; i++) {
> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>          mem = &kml->slots[i];
>          if (mem->memory_size && mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
>              kvm_slot_sync_dirty_pages(mem);
> @@ -1839,12 +1899,9 @@ void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
>  {
>      int i;
>  
> -    kml->slots = g_new0(KVMSlot, s->nr_slots);
>      kml->as_id = as_id;
>  
> -    for (i = 0; i < s->nr_slots; i++) {
> -        kml->slots[i].slot = i;
> -    }
> +    kvm_slots_grow(kml, KVM_MEMSLOTS_NR_ALLOC_DEFAULT);
>  
>      QSIMPLEQ_INIT(&kml->transaction_add);
>      QSIMPLEQ_INIT(&kml->transaction_del);
> @@ -2461,6 +2518,18 @@ static int kvm_init(MachineState *ms)
>          s->nr_slots = 32;
>      }

If !s->nr_slots, this 32 here^ will always be smaller than 16, so the
code below will always trigger.

>  
> +    /*
> +     * A VM will at least require a few memslots to work, or it can even
> +     * fail to boot.  Make sure the supported value is always at least
> +     * larger than what we will initially allocate.

The commit message says 16 was chosen to cover basic usage, which is
fine. But here we're disallowing anything smaller. Shouldn't QEMU always
respect what KVM decided? Of course, setting aside bugs or other
scenarios that could result in the ioctl returning 0. Could some kernel
implementation at some point want to reduce the max number of memslots
and then get effectively denied because QEMU thinks otherwise?

> +     */
> +    if (s->nr_slots < KVM_MEMSLOTS_NR_ALLOC_DEFAULT) {
> +        ret = -EINVAL;
> +        fprintf(stderr, "KVM max supported number of slots (%d) too small\n",
> +                s->nr_slots);
> +        goto err;
> +    }
> +
>      s->nr_as = kvm_check_extension(s, KVM_CAP_MULTI_ADDRESS_SPACE);
>      if (s->nr_as <= 1) {
>          s->nr_as = 1;
> diff --git a/accel/kvm/trace-events b/accel/kvm/trace-events
> index 37626c1ac5..ad2ae6fca5 100644
> --- a/accel/kvm/trace-events
> +++ b/accel/kvm/trace-events
> @@ -36,3 +36,4 @@ kvm_io_window_exit(void) ""
>  kvm_run_exit_system_event(int cpu_index, uint32_t event_type) "cpu_index %d, system_even_type %"PRIu32
>  kvm_convert_memory(uint64_t start, uint64_t size, const char *msg) "start 0x%" PRIx64 " size 0x%" PRIx64 " %s"
>  kvm_memory_fault(uint64_t start, uint64_t size, uint64_t flags) "start 0x%" PRIx64 " size 0x%" PRIx64 " flags 0x%" PRIx64
> +kvm_slots_grow(unsigned int old, unsigned int new) "%u -> %u"

Fabiano Rosas <farosas@suse.de> writes:

> Peter Xu <peterx@redhat.com> writes:
>
>> Zhiyi reported an infinite loop issue in VFIO use case.  The cause of that
>> was a separate discussion, however during that I found a regression of
>> dirty sync slowness when profiling.
>>
>> Each KVMMemoryListerner maintains an array of kvm memslots.  Currently it's
>> statically allocated to be the max supported by the kernel.  However after
>> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
>> the max supported memslots reported now grows to some number large enough
>> so that it may not be wise to always statically allocate with the max
>> reported.
>>
>> What's worse, QEMU kvm code still walks all the allocated memslots entries
>> to do any form of lookups.  It can drastically slow down all memslot
>> operations because each of such loop can run over 32K times on the new
>> kernels.
>>
>> Fix this issue by making the memslots to be allocated dynamically.
>>
>> Here the initial size was set to 16 because it should cover the basic VM
>> usages, so that the hope is the majority VM use case may not even need to
>> grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
>> it'll consume 9 memslots), however not too large to waste memory.
>>
>> There can also be even better way to address this, but so far this is the
>> simplest and should be already better even than before we grow the max
>> supported memslots.  For example, in the case of above issue when VFIO was
>> attached on a 32GB system, there are only ~10 memslots used.  So it could
>> be good enough as of now.
>>
>> In the above VFIO context, measurement shows that the precopy dirty sync
>> shrinked from ~86ms to ~3ms after this patch applied.  It should also apply
>> to any KVM enabled VM even without VFIO.
>>
>> NOTE: we don't have a FIXES tag for this patch because there's no real
>> commit that regressed this in QEMU. Such behavior existed for a long time,
>> but only start to be a problem when the kernel reports very large
>> nr_slots_max value.  However that's pretty common now (the kernel change
>> was merged in 2021) so we attached cc:stable because we'll want this change
>> to be backported to stable branches.
>>
>> Cc: qemu-stable <qemu-stable@nongnu.org>
>> Reported-by: Zhiyi Guo <zhguo@redhat.com>
>> Tested-by: Zhiyi Guo <zhguo@redhat.com>
>> Signed-off-by: Peter Xu <peterx@redhat.com>
>> ---
>>  include/sysemu/kvm_int.h |  1 +
>>  accel/kvm/kvm-all.c      | 99 ++++++++++++++++++++++++++++++++++------
>>  accel/kvm/trace-events   |  1 +
>>  3 files changed, 86 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
>> index 1d8fb1473b..48e496b3d4 100644
>> --- a/include/sysemu/kvm_int.h
>> +++ b/include/sysemu/kvm_int.h
>> @@ -46,6 +46,7 @@ typedef struct KVMMemoryListener {
>>      MemoryListener listener;
>>      KVMSlot *slots;
>>      unsigned int nr_used_slots;
>> +    unsigned int nr_slots_allocated;
>>      int as_id;
>>      QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_add;
>>      QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_del;
>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>> index 75d11a07b2..c51a3f18db 100644
>> --- a/accel/kvm/kvm-all.c
>> +++ b/accel/kvm/kvm-all.c
>> @@ -69,6 +69,9 @@
>>  #define KVM_GUESTDBG_BLOCKIRQ 0
>>  #endif
>>  
>> +/* Default num of memslots to be allocated when VM starts */
>> +#define  KVM_MEMSLOTS_NR_ALLOC_DEFAULT                      16
>> +
>>  struct KVMParkedVcpu {
>>      unsigned long vcpu_id;
>>      int kvm_fd;
>> @@ -165,6 +168,57 @@ void kvm_resample_fd_notify(int gsi)
>>      }
>>  }
>>  
>> +/**
>> + * kvm_slots_grow(): Grow the slots[] array in the KVMMemoryListener
>> + *
>> + * @kml: The KVMMemoryListener* to grow the slots[] array
>> + * @nr_slots_new: The new size of slots[] array
>> + *
>> + * Returns: True if the array grows larger, false otherwise.
>> + */
>> +static bool kvm_slots_grow(KVMMemoryListener *kml, unsigned int nr_slots_new)
>> +{
>> +    unsigned int i, cur = kml->nr_slots_allocated;
>> +    KVMSlot *slots;
>> +
>> +    if (nr_slots_new > kvm_state->nr_slots) {
>> +        nr_slots_new = kvm_state->nr_slots;
>> +    }
>> +
>> +    if (cur >= nr_slots_new) {
>> +        /* Big enough, no need to grow, or we reached max */
>> +        return false;
>> +    }
>> +
>> +    if (cur == 0) {
>> +        slots = g_new0(KVMSlot, nr_slots_new);
>> +    } else {
>> +        assert(kml->slots);
>> +        slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
>> +        /*
>> +         * g_renew() doesn't initialize extended buffers, however kvm
>> +         * memslots require fields to be zero-initialized. E.g. pointers,
>> +         * memory_size field, etc.
>> +         */
>> +        memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
>> +    }
>> +
>> +    for (i = cur; i < nr_slots_new; i++) {
>> +        slots[i].slot = i;
>> +    }
>> +
>> +    kml->slots = slots;
>> +    kml->nr_slots_allocated = nr_slots_new;
>> +    trace_kvm_slots_grow(cur, nr_slots_new);
>> +
>> +    return true;
>> +}
>> +
>> +static bool kvm_slots_double(KVMMemoryListener *kml)
>> +{
>> +    return kvm_slots_grow(kml, kml->nr_slots_allocated * 2);
>> +}
>> +
>>  unsigned int kvm_get_max_memslots(void)
>>  {
>>      KVMState *s = KVM_STATE(current_accel());
>> @@ -193,15 +247,26 @@ unsigned int kvm_get_free_memslots(void)
>>  /* Called with KVMMemoryListener.slots_lock held */
>>  static KVMSlot *kvm_get_free_slot(KVMMemoryListener *kml)
>>  {
>> -    KVMState *s = kvm_state;
>> +    unsigned int n;
>>      int i;
>>  
>> -    for (i = 0; i < s->nr_slots; i++) {
>> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>>          if (kml->slots[i].memory_size == 0) {
>>              return &kml->slots[i];
>>          }
>>      }
>>  
>> +    /*
>> +     * If no free slots, try to grow first by doubling.  Cache the old size
>> +     * here to avoid another round of search: if the grow succeeded, it
>> +     * means slots[] now must have the existing "n" slots occupied,
>> +     * followed by one or more free slots starting from slots[n].
>> +     */
>> +    n = kml->nr_slots_allocated;
>> +    if (kvm_slots_double(kml)) {
>> +        return &kml->slots[n];
>> +    }
>> +
>>      return NULL;
>>  }
>>  
>> @@ -222,10 +287,9 @@ static KVMSlot *kvm_lookup_matching_slot(KVMMemoryListener *kml,
>>                                           hwaddr start_addr,
>>                                           hwaddr size)
>>  {
>> -    KVMState *s = kvm_state;
>>      int i;
>>  
>> -    for (i = 0; i < s->nr_slots; i++) {
>> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>>          KVMSlot *mem = &kml->slots[i];
>>  
>>          if (start_addr == mem->start_addr && size == mem->memory_size) {
>> @@ -267,7 +331,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s, void *ram,
>>      int i, ret = 0;
>>  
>>      kvm_slots_lock();
>> -    for (i = 0; i < s->nr_slots; i++) {
>> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>>          KVMSlot *mem = &kml->slots[i];
>>  
>>          if (ram >= mem->ram && ram < mem->ram + mem->memory_size) {
>> @@ -1071,7 +1135,7 @@ static int kvm_physical_log_clear(KVMMemoryListener *kml,
>>  
>>      kvm_slots_lock();
>>  
>> -    for (i = 0; i < s->nr_slots; i++) {
>> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>>          mem = &kml->slots[i];
>>          /* Discard slots that are empty or do not overlap the section */
>>          if (!mem->memory_size ||
>> @@ -1719,12 +1783,8 @@ static void kvm_log_sync_global(MemoryListener *l, bool last_stage)
>>      /* Flush all kernel dirty addresses into KVMSlot dirty bitmap */
>>      kvm_dirty_ring_flush();
>>  
>> -    /*
>> -     * TODO: make this faster when nr_slots is big while there are
>> -     * only a few used slots (small VMs).
>> -     */
>>      kvm_slots_lock();
>> -    for (i = 0; i < s->nr_slots; i++) {
>> +    for (i = 0; i < kml->nr_slots_allocated; i++) {
>>          mem = &kml->slots[i];
>>          if (mem->memory_size && mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
>>              kvm_slot_sync_dirty_pages(mem);
>> @@ -1839,12 +1899,9 @@ void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
>>  {
>>      int i;
>>  
>> -    kml->slots = g_new0(KVMSlot, s->nr_slots);
>>      kml->as_id = as_id;
>>  
>> -    for (i = 0; i < s->nr_slots; i++) {
>> -        kml->slots[i].slot = i;
>> -    }
>> +    kvm_slots_grow(kml, KVM_MEMSLOTS_NR_ALLOC_DEFAULT);
>>  
>>      QSIMPLEQ_INIT(&kml->transaction_add);
>>      QSIMPLEQ_INIT(&kml->transaction_del);
>> @@ -2461,6 +2518,18 @@ static int kvm_init(MachineState *ms)
>>          s->nr_slots = 32;
>>      }
>
> If !s->nr_slots, this 32 here^ will always be smaller than 16, so the
> code below will always trigger.

Hehe, nevermind, crossed wires in my brain.

>>  
>> +    /*
>> +     * A VM will at least require a few memslots to work, or it can even
>> +     * fail to boot.  Make sure the supported value is always at least
>> +     * larger than what we will initially allocate.
>
> The commit message says 16 was chosen to cover basic usage, which is
> fine. But here we're disallowing anything smaller. Shouldn't QEMU always
> respect what KVM decided? Of course, setting aside bugs or other
> scenarios that could result in the ioctl returning 0. Could some kernel
> implementation at some point want to reduce the max number of memslots
> and then get effectively denied because QEMU thinks otherwise?
>
>> +     */
>> +    if (s->nr_slots < KVM_MEMSLOTS_NR_ALLOC_DEFAULT) {
>> +        ret = -EINVAL;
>> +        fprintf(stderr, "KVM max supported number of slots (%d) too small\n",
>> +                s->nr_slots);
>> +        goto err;
>> +    }
>> +
>>      s->nr_as = kvm_check_extension(s, KVM_CAP_MULTI_ADDRESS_SPACE);
>>      if (s->nr_as <= 1) {
>>          s->nr_as = 1;
>> diff --git a/accel/kvm/trace-events b/accel/kvm/trace-events
>> index 37626c1ac5..ad2ae6fca5 100644
>> --- a/accel/kvm/trace-events
>> +++ b/accel/kvm/trace-events
>> @@ -36,3 +36,4 @@ kvm_io_window_exit(void) ""
>>  kvm_run_exit_system_event(int cpu_index, uint32_t event_type) "cpu_index %d, system_even_type %"PRIu32
>>  kvm_convert_memory(uint64_t start, uint64_t size, const char *msg) "start 0x%" PRIx64 " size 0x%" PRIx64 " %s"
>>  kvm_memory_fault(uint64_t start, uint64_t size, uint64_t flags) "start 0x%" PRIx64 " size 0x%" PRIx64 " flags 0x%" PRIx64
>> +kvm_slots_grow(unsigned int old, unsigned int new) "%u -> %u"

On Mon, Sep 16, 2024 at 02:52:04PM -0300, Fabiano Rosas wrote:
> >>  
> >> +    /*
> >> +     * A VM will at least require a few memslots to work, or it can even
> >> +     * fail to boot.  Make sure the supported value is always at least
> >> +     * larger than what we will initially allocate.
> >
> > The commit message says 16 was chosen to cover basic usage, which is
> > fine. But here we're disallowing anything smaller. Shouldn't QEMU always
> > respect what KVM decided? Of course, setting aside bugs or other
> > scenarios that could result in the ioctl returning 0. Could some kernel
> > implementation at some point want to reduce the max number of memslots
> > and then get effectively denied because QEMU thinks otherwise?

I'd say it's unlikely to happen, but indeed failing it here might be based
too much over the artificial KVM_MEMSLOTS_NR_ALLOC_DEFAULT I came up with.

If this check is removed, I suppose we're still fine.  So it means later
when qemu initializes kvm memslots here:

    kvm_slots_grow(kml, KVM_MEMSLOTS_NR_ALLOC_DEFAULT);

It'll be throttled by whatever kvm specified lower than 16. Then if
memslots are not enough, we'll fail at memslot allocation whenever
requested more than what kvm offers:

    fprintf(stderr, "%s: no free slot available\n", __func__);
    abort();

Yeah, maybe it is better to fail here.. Even though I think we'll never use
it, but still good to remove some lines if they're not needed.  Let me
remove this check in my next post.

Thanks,