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
