linux-kernel - Re: [PATCH v5 1/2] KVM: x86: Advance guest TSC after deep suspend.

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Message-ID: <aAcfcB8ZyBuz7t7J@google.com>
Date: Tue, 22 Apr 2025 04:47:44 +0000
From: Tzung-Bi Shih <tzungbi@...nel.org>
To: Suleiman Souhlal <suleiman@...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>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, ssouhlal@...ebsd.org
Subject: Re: [PATCH v5 1/2] KVM: x86: Advance guest TSC after deep suspend.

On Tue, Mar 25, 2025 at 01:13:49PM +0900, Suleiman Souhlal wrote:
> 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 with comparing `date` before and after suspend-to-RAM[1]:
  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>

[1]: https://www.kernel.org/doc/Documentation/power/states.txt

Some non-functional comments inline below.

> @@ -4971,7 +4971,37 @@ 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)) {
> -		adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment);
> +		unsigned long flags;
> +		struct kvm *kvm;
> +		bool advance;
> +		u64 kernel_ns, l1_tsc, offset, tsc_now;
> +
> +		kvm = vcpu->kvm;

It will be more clear (at least to me) if moving the statement to its declaration:
  struct kvm *kvm = vcpu->kvm;

Other than that, the following code should better utilitize the local
variable, e.g. s/vcpu->kvm/kvm/g.

> +		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,
> +			    vcpu->kvm->arch.kvmclock_offset +
                            ^^^^^^^^^
                            kvm

> +			    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,
> +			    vcpu->kvm->arch.cur_tsc_offset);
                            ^^^^^^^^^
			    kvm

> +		else
> +			adjust_tsc_offset_host(vcpu,
> +			    vcpu->arch.tsc_offset_adjustment);

Need braces in `else if` and `else` cases [2].

[2]: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces


> @@ -12640,6 +12670,7 @@ int kvm_arch_enable_virtualization_cpu(void)
>  				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
>  			}
>  
> +			kvm->arch.host_was_suspended = 1;

Given that it is a bool, how about use `true`?