Re: [PATCH v8 4/7] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Sep 16, 2021 at 06:15:35PM +0000, Oliver Upton wrote:
> Handling the migration of TSCs correctly is difficult, in part because
> Linux does not provide userspace with the ability to retrieve a (TSC,
> realtime) clock pair for a single instant in time. In lieu of a more
> convenient facility, KVM can report similar information in the kvm_clock
> structure.
> 
> Provide userspace with a host TSC & realtime pair iff the realtime clock
> is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
> realtime value, advance the KVM clock by the amount of elapsed time. Do
> not step the KVM clock backwards, though, as it is a monotonic
> oscillator.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  Documentation/virt/kvm/api.rst  | 42 ++++++++++++++++++++++++++-------
>  arch/x86/include/asm/kvm_host.h |  3 +++
>  arch/x86/kvm/x86.c              | 36 +++++++++++++++++++++-------
>  include/uapi/linux/kvm.h        |  7 +++++-
>  4 files changed, 70 insertions(+), 18 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a6729c8cf063..d0b9c986cf6c 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -993,20 +993,34 @@ such as migration.
>  When KVM_CAP_ADJUST_CLOCK is passed to KVM_CHECK_EXTENSION, it returns the
>  set of bits that KVM can return in struct kvm_clock_data's flag member.
>  
> -The only flag defined now is KVM_CLOCK_TSC_STABLE.  If set, the returned
> -value is the exact kvmclock value seen by all VCPUs at the instant
> -when KVM_GET_CLOCK was called.  If clear, the returned value is simply
> -CLOCK_MONOTONIC plus a constant offset; the offset can be modified
> -with KVM_SET_CLOCK.  KVM will try to make all VCPUs follow this clock,
> -but the exact value read by each VCPU could differ, because the host
> -TSC is not stable.
> +FLAGS:
> +
> +KVM_CLOCK_TSC_STABLE.  If set, the returned value is the exact kvmclock
> +value seen by all VCPUs at the instant when KVM_GET_CLOCK was called.
> +If clear, the returned value is simply CLOCK_MONOTONIC plus a constant
> +offset; the offset can be modified with KVM_SET_CLOCK.  KVM will try
> +to make all VCPUs follow this clock, but the exact value read by each
> +VCPU could differ, because the host TSC is not stable.
> +
> +KVM_CLOCK_REALTIME.  If set, the `realtime` field in the kvm_clock_data
> +structure is populated with the value of the host's real time
> +clocksource at the instant when KVM_GET_CLOCK was called. If clear,
> +the `realtime` field does not contain a value.
> +
> +KVM_CLOCK_HOST_TSC.  If set, the `host_tsc` field in the kvm_clock_data
> +structure is populated with the value of the host's timestamp counter (TSC)
> +at the instant when KVM_GET_CLOCK was called. If clear, the `host_tsc` field
> +does not contain a value.

If the host TSCs are not stable, then KVM_CLOCK_HOST_TSC bit (and
host_tsc field) are ambiguous. Shouldnt exposing them be conditional on 
stable TSC for the host ?