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

Paolo,

On Sun, Aug 15, 2021 at 5:11 PM Oliver Upton [off-list ref] 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 <redacted>

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

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