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

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Message-ID: <CABCjUKAT03ficWMg+mkDWev15x+E3YDin1_9VkJBgb4g-F4UmA@mail.gmail.com>
Date: Fri, 18 Jul 2025 12:09:57 +0900
From: Suleiman Souhlal <suleiman@...gle.com>
To: John Stultz <jstultz@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Chao Gao <chao.gao@...el.com>, 
	David Woodhouse <dwmw2@...radead.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Tzung-Bi Shih <tzungbi@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, ssouhlal@...ebsd.org
Subject: Re: [PATCH v7 1/3] KVM: x86: Advance guest TSC after deep suspend.

On Fri, Jul 18, 2025 at 5:43 AM John Stultz <jstultz@...gle.com> wrote:
>
> On Sun, Jul 13, 2025 at 8:37 PM Suleiman Souhlal <suleiman@...gle.com> 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@...gle.com>
> > Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
> > ---
> >  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