linux-kernel - Re: [PATCH v2] KVM: x86/tsc: Use calculated tsc_offset before matching the fist vcpu's tsc

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Message-ID: <fe59c490-da64-10ae-7f94-88d842e9f839@redhat.com>
Date:   Sat, 29 Jul 2023 16:53:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86/tsc: Use calculated tsc_offset before
 matching the fist vcpu's tsc

On 7/24/23 09:35, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> Avoid using kvm->arch.cur_tsc_offset until tsc sync is actually needed.
> 
> When the vcpu is created for the first time, its tsc is 0, and KVM
> calculates its tsc_offset according to kvm_compute_l1_tsc_offset().
> The userspace will then set the first non-zero tsc expected value for the
> first guest vcpu, at this time there is no need to play the tsc synchronize
> mechanism, the KVM should continue to write tsc_offset based on previously
> used kvm_compute_l1_tsc_offset().
> 
> If the tsc synchronization mechanism is incorrectly applied at this point,
> KVM will use the rewritten offset of the kvm->arch.cur_tsc_offset (on
> tsc_stable machines) to write tsc_offset, which is not in line with the
> expected tsc of the user space.
> 
> Based on the current code, the vcpu's tsc_offset is not configured as
> expected, resulting in significant guest service response latency, which
> is observed in our production environment.
> 
> Applying the tsc synchronization logic after the vcpu's tsc_generation
> and KVM's cur_tsc_generation have completed their first match and started
> keeping tracked helps to fix this issue, which also does not break any
> existing guest tsc test cases.
> 
> Reported-by: Yong He <alexyonghe@...cent.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Suggested-by: Oliver Upton <oliver.upton@...ux.dev>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
> V1 -> V2 Changelog:
> - Test the 'kvm_vcpu_has_run(vcpu)' proposal; (Sean)
> - Test the kvm->arch.user_changed_tsc proposal; (Oliver)
> V1: https://lore.kernel.org/kvm/20230629164838.66847-1-likexu@tencent.com/
> 
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a6b9bea62fb8..4724dacea2df 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2721,7 +2721,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
>   			 * kvm_clock stable after CPU hotplug
>   			 */
>   			synchronizing = true;
> -		} else {
> +		} else if (kvm->arch.nr_vcpus_matched_tsc) {
>   			u64 tsc_exp = kvm->arch.last_tsc_write +
>   						nsec_to_cycles(vcpu, elapsed);
>   			u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
> 

I am not sure, when will kvm->arch.nr_vcpus_matched_tsc ever be nonzero 
again once a new generation starts?  If this patch ever causes 
synchronizing to be false, matched will also be false when calling 
__kvm_synchronize_tsc.

What was wrong with Oliver's proposed patch?

Paolo