On Fri, Jul 18, 2025 at 5:43 AM John Stultz <jstultz@xxxxxxxxxx> wrote:
>
> 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 for the good suggestions. I'll incorporate them into v8.
-- Suleiman
