| Message ID | 20241010182427.1434605-35-seanjc@google.com |
|---|---|
| State | Accepted |
| Headers | show |
| Series | KVM: Stop grabbing references to PFNMAP'd pages | expand |
On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote: > When creating a memory map for read, don't request a writable pfn from the > primary MMU. While creating read-only mappings can be theoretically slower, > as they don't play nice with fast GUP due to the need to break CoW before > mapping the underlying PFN, practically speaking, creating a mapping isn't > a super hot path, and getting a writable mapping for reading is weird and > confusing. > > Tested-by: Alex Bennée <alex.bennee@linaro.org> > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > virt/kvm/kvm_main.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 080740f65061..b845e9252633 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map, > struct kvm_follow_pfn kfp = { > .slot = gfn_to_memslot(vcpu->kvm, gfn), > .gfn = gfn, > - .flags = FOLL_WRITE, > + .flags = writable ? FOLL_WRITE : 0, > .refcounted_page = &map->pinned_page, > .pin = true, > }; When writable is false, could we set ".pin = false," ? Also not sure if ".map_writable = NULL" is missing.
On Mon, Oct 21, 2024, Yan Zhao wrote: > On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote: > > When creating a memory map for read, don't request a writable pfn from the > > primary MMU. While creating read-only mappings can be theoretically slower, > > as they don't play nice with fast GUP due to the need to break CoW before > > mapping the underlying PFN, practically speaking, creating a mapping isn't > > a super hot path, and getting a writable mapping for reading is weird and > > confusing. > > > > Tested-by: Alex Bennée <alex.bennee@linaro.org> > > Signed-off-by: Sean Christopherson <seanjc@google.com> > > --- > > virt/kvm/kvm_main.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index 080740f65061..b845e9252633 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map, > > struct kvm_follow_pfn kfp = { > > .slot = gfn_to_memslot(vcpu->kvm, gfn), > > .gfn = gfn, > > - .flags = FOLL_WRITE, > > + .flags = writable ? FOLL_WRITE : 0, > > .refcounted_page = &map->pinned_page, > > .pin = true, > > }; > When writable is false, could we set ".pin = false," ? Hmm, maybe? I can't imagine anything would actually break, but unless FOLL_PIN implies writing, my preference would still be to pin the page so that KVM always pins when accessing the actual data of a page. > Also not sure if ".map_writable = NULL" is missing. Doh, my previous response was slightly wrong, it's implicitly initialized to NULL, not false. I forgot map_writable is a pointer to a bool.
On Mon, Oct 21, 2024 at 11:13:08AM -0700, Sean Christopherson wrote: > On Mon, Oct 21, 2024, Yan Zhao wrote: > > On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote: > > > When creating a memory map for read, don't request a writable pfn from the > > > primary MMU. While creating read-only mappings can be theoretically slower, > > > as they don't play nice with fast GUP due to the need to break CoW before > > > mapping the underlying PFN, practically speaking, creating a mapping isn't > > > a super hot path, and getting a writable mapping for reading is weird and > > > confusing. > > > > > > Tested-by: Alex Bennée <alex.bennee@linaro.org> > > > Signed-off-by: Sean Christopherson <seanjc@google.com> > > > --- > > > virt/kvm/kvm_main.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > > index 080740f65061..b845e9252633 100644 > > > --- a/virt/kvm/kvm_main.c > > > +++ b/virt/kvm/kvm_main.c > > > @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map, > > > struct kvm_follow_pfn kfp = { > > > .slot = gfn_to_memslot(vcpu->kvm, gfn), > > > .gfn = gfn, > > > - .flags = FOLL_WRITE, > > > + .flags = writable ? FOLL_WRITE : 0, > > > .refcounted_page = &map->pinned_page, > > > .pin = true, > > > }; > > When writable is false, could we set ".pin = false," ? > > Hmm, maybe? I can't imagine anything would actually break, but unless FOLL_PIN > implies writing, my preference would still be to pin the page so that KVM always > pins when accessing the actual data of a page. Ok. So setting .pin = true here is because of KVM direct access, which does not check mmu notifier's invalidation callback.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 080740f65061..b845e9252633 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map, struct kvm_follow_pfn kfp = { .slot = gfn_to_memslot(vcpu->kvm, gfn), .gfn = gfn, - .flags = FOLL_WRITE, + .flags = writable ? FOLL_WRITE : 0, .refcounted_page = &map->pinned_page, .pin = true, };