| Message ID | 20240503181734.1467938-1-dmatlack@google.com |
|---|---|
| Headers | show |
| Series | KVM: Set vcpu->preempted/ready iff scheduled out while running | expand |
On Fri, 03 May 2024 11:17:31 -0700, David Matlack wrote: > This series changes KVM to mark a vCPU as preempted/ready if-and-only-if > it's scheduled out while running. i.e. Do not mark a vCPU > preempted/ready if it's scheduled out during a non-KVM_RUN ioctl() or > when userspace is doing KVM_RUN with immediate_exit=true. > > This is a logical extension of commit 54aa83c90198 ("KVM: x86: do not > set st->preempted when going back to user space"), which stopped > marking a vCPU as preempted when returning to userspace. But if userspace > invokes a KVM vCPU ioctl() that gets preempted, the vCPU will be marked > preempted/ready. This is arguably incorrect behavior since the vCPU was > not actually preempted while the guest was running, it was preempted > while doing something on behalf of userspace. > > [...] Applied to kvm-x86 generic, with minor changelog tweaks (me thinks you've been away from upstream too long ;-) ). Thanks! [1/3] KVM: Introduce vcpu->wants_to_run https://github.com/kvm-x86/linux/commit/a6816314af57 [2/3] KVM: Ensure new code that references immediate_exit gets extra scrutiny https://github.com/kvm-x86/linux/commit/4b23e0c199b2 [3/3] KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running https://github.com/kvm-x86/linux/commit/118964562969 -- https://github.com/kvm-x86/linux/tree/next
On Tue, Jun 18, 2024 at 2:41 PM Sean Christopherson <seanjc@google.com> wrote: > > On Fri, 03 May 2024 11:17:31 -0700, David Matlack wrote: > > This series changes KVM to mark a vCPU as preempted/ready if-and-only-if > > it's scheduled out while running. i.e. Do not mark a vCPU > > preempted/ready if it's scheduled out during a non-KVM_RUN ioctl() or > > when userspace is doing KVM_RUN with immediate_exit=true. > > > > This is a logical extension of commit 54aa83c90198 ("KVM: x86: do not > > set st->preempted when going back to user space"), which stopped > > marking a vCPU as preempted when returning to userspace. But if userspace > > invokes a KVM vCPU ioctl() that gets preempted, the vCPU will be marked > > preempted/ready. This is arguably incorrect behavior since the vCPU was > > not actually preempted while the guest was running, it was preempted > > while doing something on behalf of userspace. > > > > [...] > > Applied to kvm-x86 generic, with minor changelog tweaks (me thinks you've been > away from upstream too long ;-) ). Thanks! Thanks for the cleanups. Looks like you replaced "[Tt]his commit" throughout. Anything else (so I can avoid the same mistakes in the future)? > > [1/3] KVM: Introduce vcpu->wants_to_run > https://github.com/kvm-x86/linux/commit/a6816314af57 > [2/3] KVM: Ensure new code that references immediate_exit gets extra scrutiny > https://github.com/kvm-x86/linux/commit/4b23e0c199b2 > [3/3] KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running > https://github.com/kvm-x86/linux/commit/118964562969 > > -- > https://github.com/kvm-x86/linux/tree/next
On Mon, Jul 01, 2024, David Matlack wrote: > On Tue, Jun 18, 2024 at 2:41 PM Sean Christopherson <seanjc@google.com> wrote: > > > > On Fri, 03 May 2024 11:17:31 -0700, David Matlack wrote: > > > This series changes KVM to mark a vCPU as preempted/ready if-and-only-if > > > it's scheduled out while running. i.e. Do not mark a vCPU > > > preempted/ready if it's scheduled out during a non-KVM_RUN ioctl() or > > > when userspace is doing KVM_RUN with immediate_exit=true. > > > > > > This is a logical extension of commit 54aa83c90198 ("KVM: x86: do not > > > set st->preempted when going back to user space"), which stopped > > > marking a vCPU as preempted when returning to userspace. But if userspace > > > invokes a KVM vCPU ioctl() that gets preempted, the vCPU will be marked > > > preempted/ready. This is arguably incorrect behavior since the vCPU was > > > not actually preempted while the guest was running, it was preempted > > > while doing something on behalf of userspace. > > > > > > [...] > > > > Applied to kvm-x86 generic, with minor changelog tweaks (me thinks you've been > > away from upstream too long ;-) ). Thanks! > > Thanks for the cleanups. Looks like you replaced "[Tt]his commit" > throughout. Anything else (so I can avoid the same mistakes in the > future)? I don't think so? The "This commit" stuff is the only thing that I remember, so any other tweaks can't be that important :-)
This series changes KVM to mark a vCPU as preempted/ready if-and-only-if it's scheduled out while running. i.e. Do not mark a vCPU preempted/ready if it's scheduled out during a non-KVM_RUN ioctl() or when userspace is doing KVM_RUN with immediate_exit=true. This is a logical extension of commit 54aa83c90198 ("KVM: x86: do not set st->preempted when going back to user space"), which stopped marking a vCPU as preempted when returning to userspace. But if userspace invokes a KVM vCPU ioctl() that gets preempted, the vCPU will be marked preempted/ready. This is arguably incorrect behavior since the vCPU was not actually preempted while the guest was running, it was preempted while doing something on behalf of userspace. In practice, this avoids KVM dirtying guest memory via the steal time page after userspace has paused vCPUs, e.g. for Live Migration, which allows userspace to collect the final dirty bitmap before or in parallel with saving vCPU state without having to worry about saving vCPU state triggering writes to guest memory. Patch 1 introduces vcpu->wants_to_run to allow KVM to detect when a vCPU is in its core run loop. Patch 2 renames immediated_exit to immediated_exit__unsafe within KVM to ensure that any new references get extra scrutiny. Patch 3 perform leverages vcpu->wants_to_run to contrain when vcpu->preempted and vcpu->ready are set. v3: - Use READ_ONCE() to read immediate_exit [Sean] - Replace use of immediate_exit with !wants_to_run to avoid TOCTOU [Sean] - Hide/Rename immediate_exit in KVM to harden against TOCTOU bugs [Sean] v2: https://lore.kernel.org/kvm/20240307163541.92138-1-dmatlack@google.com/ - Drop Google-specific "PRODKERNEL: " shortlog prefix [me] v1: https://lore.kernel.org/kvm/20231218185850.1659570-1-dmatlack@google.com/ David Matlack (3): KVM: Introduce vcpu->wants_to_run KVM: Ensure new code that references immediate_exit gets extra scrutiny KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running arch/arm64/kvm/arm.c | 2 +- arch/loongarch/kvm/vcpu.c | 2 +- arch/mips/kvm/mips.c | 2 +- arch/powerpc/kvm/powerpc.c | 2 +- arch/riscv/kvm/vcpu.c | 2 +- arch/s390/kvm/kvm-s390.c | 2 +- arch/x86/kvm/x86.c | 4 ++-- include/linux/kvm_host.h | 1 + include/uapi/linux/kvm.h | 15 ++++++++++++++- virt/kvm/kvm_main.c | 5 ++++- 10 files changed, 27 insertions(+), 10 deletions(-) base-commit: 296655d9bf272cfdd9d2211d099bcb8a61b93037