Re: [PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace

[Oliver Upton <oupton@google.com>]

Paolo,

On Sun, Aug 15, 2021 at 5:11 PM Oliver Upton <oupton@google.com> wrote:
>
> To date, VMM-directed TSC synchronization and migration has been a bit
> messy. KVM has some baked-in heuristics around TSC writes to infer if
> the VMM is attempting to synchronize. This is problematic, as it depends
> on host userspace writing to the guest's TSC within 1 second of the last
> write.
>
> A much cleaner approach to configuring the guest's views of the TSC is to
> simply migrate the TSC offset for every vCPU. Offsets are idempotent,
> and thus not subject to change depending on when the VMM actually
> reads/writes values from/to KVM. The VMM can then read the TSC once with
> KVM_GET_CLOCK to capture a (realtime, host_tsc) pair at the instant when
> the guest is paused.
>
> Cc: David Matlack <dmatlack@google.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Oliver Upton <oupton@google.com>

Could you please squash the following into this patch? We need to
advertise KVM_CAP_VCPU_ATTRIBUTES to userspace. Otherwise, happy to
resend.

Thanks,
Oliver

 arch/x86/kvm/x86.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b946430faaae..b5be1ca07704 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4070,6 +4070,7 @@ int kvm_vm_ioctl_check_extension(struct kvm
*kvm, long ext)
        case KVM_CAP_VM_COPY_ENC_CONTEXT_FROM:
        case KVM_CAP_SREGS2:
        case KVM_CAP_EXIT_ON_EMULATION_FAILURE:
+       case KVM_CAP_VCPU_ATTRIBUTES:
                r = 1;
                break;
        case KVM_CAP_EXIT_HYPERCALL:
-- 
2.33.0.rc2.250.ged5fa647cd-goog