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

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Message-ID: <CABCjUKB07exDN9nkjoio7LFct51GRaT=ZGXUq3iwoqfLog-trA@mail.gmail.com>
Date: Thu, 17 Jul 2025 15:31:19 +0900
From: Suleiman Souhlal <suleiman@...gle.com>
To: Tzung-Bi Shih <tzungbi@...nel.org>
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>, John Stultz <jstultz@...gle.com>, 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 Tue, Jul 15, 2025 at 2:29 PM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
>
> On Mon, Jul 14, 2025 at 12:36:47PM +0900, Suleiman Souhlal 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>
>
> Tested again with comparing `date` before and after suspend-to-RAM:
>   echo deep >/sys/power/mem_sleep
>   echo $(date '+%s' -d '+3 minutes') >/sys/class/rtc/rtc0/wakealarm
>   echo mem >/sys/power/state
>
> Without the patch, the guest's `date` is slower (~3 mins) than the host's
> after resuming.
>
> Tested-by: Tzung-Bi Shih <tzungbi@...nel.org>

Thanks for testing!

>
> > @@ -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 */
>
> Wondering if it needs to acquire the `tsc_write_lock`, given that:
> - The original code adjust_tsc_offset_host() doesn't acquire.  Note:
>   adjust_tsc_offset_host() eventually calls kvm_vcpu_write_tsc_offset() too.
> - Documentation/virt/kvm/locking.rst [1].
>
> [1] https://elixir.bootlin.com/linux/v6.15/source/Documentation/virt/kvm/locking.rst#L264

This is an excellent question.
I used a lock here to make sure that only one VCPU computes the offset
and that all the others reuse it.
It might be doable with atomic operations, but using a lock seemed
simpler to me.
I don't think it has to be tsc_write_lock specifically, but reusing it
for this purpose seemed appropriate to me.

Thanks,
-- Suleiman