From: Sean Christopherson <seanjc@google.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Suleiman Souhlal <suleiman@google.com>,
Anton Romanov <romanton@google.com>
Subject: Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant
Date: Fri, 25 Feb 2022 16:40:48 +0000 [thread overview]
Message-ID: <YhkGkAJtMu0epKiT@google.com> (raw)
In-Reply-To: <5b9e5a3f3d3c40afea0bc953e3967505251f3143.camel@infradead.org>
On Fri, Feb 25, 2022, David Woodhouse wrote:
> On Fri, 2022-02-25 at 01:39 +0000, Sean Christopherson wrote:
> > @@ -11160,7 +11162,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> > vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
> > kvm_vcpu_mtrr_init(vcpu);
> > vcpu_load(vcpu);
> > - kvm_set_tsc_khz(vcpu, max_tsc_khz);
> > + kvm_set_tsc_khz(vcpu, max_tsc_khz ? : tsc_khz);
> > kvm_vcpu_reset(vcpu, false);
> > kvm_init_mmu(vcpu);
> > vcpu_put(vcpu);
> >
>
> Hm, now if you hit that race you end up potentially giving *different*
> frequencies to different vCPUs in a single guest, depending on when
> they were created.
Yep. Though the race is much harder to hit (userspace vs TSC refinement). The
existing race being hit is essentially do_initcalls() vs. TSC refinement.
> How about this... (and as noted, I think I want to add an explicit KVM
> ioctl to set kvm->arch.default_tsc_khz for subsequently created vCPUs).
This wouldn't necessarily help. E.g. assuming userspace knows the actual TSC
frequency, creating a vCPU before refinement completes might put the vCPU in
"always catchup" purgatory.
To really fix the race, KVM needs a notification that refinement completed (or
failed). KVM could simply refuse to create vCPUs until it got the notification.
In the non-constant case, KVM would also need to refresh max_tsc_khz.
next prev parent reply other threads:[~2022-02-25 16:40 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-25 1:39 [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant Sean Christopherson
2022-02-25 12:10 ` Paolo Bonzini
2022-02-25 12:20 ` David Woodhouse
2022-02-25 16:21 ` Sean Christopherson
2022-02-25 16:34 ` David Woodhouse
2022-04-12 17:38 ` Anton Romanov
2022-04-13 8:31 ` Paolo Bonzini
2022-02-25 12:52 ` David Woodhouse
2022-02-25 16:40 ` Sean Christopherson [this message]
2022-02-25 16:51 ` David Woodhouse
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