Re: [PATCH v7 1/3] KVM: x86: Advance guest TSC after deep suspend.

[John Stultz]

On Sun, Jul 13, 2025 at 8:37 PM Suleiman Souhlal <suleiman@xxxxxxxxxx> wrote:
>
> Try to advance guest TSC to current time after suspend when the host
> TSCs went backwards.
>
> This makes the behavior consistent between suspends where host TSC
> resets and suspends where it doesn't, such as suspend-to-idle, where
> in the former case if the host TSC resets, the guests' would
> previously be "frozen" due to KVM's backwards TSC prevention, while
> in the latter case they would advance.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 +++
>  arch/x86/kvm/x86.c              | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7b9ccdd99f32..3650a513ba19 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1414,6 +1414,9 @@ struct kvm_arch {
>         u64 cur_tsc_offset;
>         u64 cur_tsc_generation;
>         int nr_vcpus_matched_tsc;
> +#ifdef CONFIG_X86_64
> +       bool host_was_suspended;
> +#endif
>
>         u32 default_tsc_khz;
>         bool user_set_tsc;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e21f5f2fe059..6539af701016 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5035,7 +5035,36 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>
>         /* Apply any externally detected TSC adjustments (due to suspend) */
>         if (unlikely(vcpu->arch.tsc_offset_adjustment)) {
> +#ifdef CONFIG_X86_64
> +               unsigned long flags;
> +               struct kvm *kvm;
> +               bool advance;
> +               u64 kernel_ns, l1_tsc, offset, tsc_now;
> +
> +               kvm = vcpu->kvm;
> +               advance = kvm_get_time_and_clockread(&kernel_ns, &tsc_now);
> +               raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
> +               /*
> +                * Advance the guest's TSC to current time instead of only
> +                * preventing it from going backwards, while making sure
> +                * all the vCPUs use the same offset.
> +                */
> +               if (kvm->arch.host_was_suspended && advance) {
> +                       l1_tsc = nsec_to_cycles(vcpu,
> +                                               kvm->arch.kvmclock_offset + kernel_ns);
> +                       offset = kvm_compute_l1_tsc_offset(vcpu, l1_tsc);
> +                       kvm->arch.cur_tsc_offset = offset;
> +                       kvm_vcpu_write_tsc_offset(vcpu, offset);
> +               } else if (advance) {
> +                       kvm_vcpu_write_tsc_offset(vcpu, kvm->arch.cur_tsc_offset);
> +               } else {
> +                       adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment);
> +               }
> +               kvm->arch.host_was_suspended = false;
> +               raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
> +#else
>                 adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment);
> +#endif /* CONFIG_X86_64 */

Just style wise, it seems like renaming adjust_tsc_offset_host() to
__adjust_tsc_offset_host(), and then moving the ifdefed logic into a
new adjust_tsc_offset_host() implementation might be cleaner?
Then you could have:

#ifdef COFNIG_X86_64
static inline void adjust_tsc_offset_host(...)
{
/* added logic above */
}
#else
static inline void adjust_tsc_offset_host(...)
{
    __adjust_tsc_offset_host(...);
}
#endif

>                 vcpu->arch.tsc_offset_adjustment = 0;
>                 kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>         }
> @@ -12729,6 +12758,9 @@ int kvm_arch_enable_virtualization_cpu(void)
>                                 kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
>                         }
>
> +#ifdef CONFIG_X86_64
> +                       kvm->arch.host_was_suspended = true;
> +#endif

Similarly I'd wrap this in a:

#ifdef CONFIG_x86_64
static inline void kvm_set_host_was_suspended(*kvm)
{
    kvm->arch.host_was_suspended = true;
}
#else
static inline void kvm_set_host_was_suspended(*kvm)
{
}
#endif

then call kvm_set_host_was_suspended(kvm) unconditionally in the logic above.

thanks
-john