From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
Maxim Levitsky <mlevitsk@redhat.com>
Cc: 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
Subject: Re: [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes
Date: Fri, 15 Oct 2021 18:23:40 +0200 [thread overview]
Message-ID: <5faa7e49-9eb6-a075-982a-aa7947a5a3d6@redhat.com> (raw)
In-Reply-To: <YWmpKTk/7MOCzm15@google.com>
On 15/10/21 18:15, Sean Christopherson wrote:
>>
>> - now vCPU1 finally starts running the page fault code.
>>
>> - vCPU1 AVIC is still enabled
>> (because vCPU1 never handled KVM_REQ_APICV_UPDATE),
>> so the page fault code will populate the SPTE.
> But vCPU1 won't install the SPTE if it loses the race to acquire mmu_lock, because
> kvm_zap_gfn_range() bumps the notifier sequence and so vCPU1 will retry the fault.
> If vCPU1 wins the race, i.e. sees the same sequence number, then the zap is
> guaranteed to find the newly-installed SPTE.
>
> And IMO, retrying is the desired behavior. Installing a SPTE based on the global
> state works, but it's all kinds of weird to knowingly take an action the directly
> contradicts the current vCPU state.
I think both of you are correct. :)
Installing a SPTE based on global state is weird because this is a vCPU
action; installing it based on vCPU state is weird because it is
knowingly out of date. I tend to be more on Maxim's side, but that may
be simply because I have reviewed the code earlier and the various
interleavings are still somewhere in my brain.
It certainly deserves a comment though. The behavior wrt the sequence
number is particularly important if you use the vCPU state, but it's
worth pointing out even with the current code; this exchange shows that
it can be confusing.
Paolo
next prev parent reply other threads:[~2021-10-15 16:23 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-09 1:01 [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes Sean Christopherson
2021-10-09 1:01 ` [PATCH 1/2] KVM: x86/mmu: Use vCPU's APICv status when handling APIC_ACCESS memslot Sean Christopherson
2021-10-10 12:47 ` Maxim Levitsky
2021-10-09 1:01 ` [PATCH 2/2] KVM: x86: Simplify APICv update request logic Sean Christopherson
2021-10-10 12:49 ` Maxim Levitsky
2021-10-11 17:55 ` Sean Christopherson
2021-10-10 12:37 ` [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes Maxim Levitsky
2021-10-11 14:27 ` Sean Christopherson
2021-10-11 16:58 ` Sean Christopherson
2021-10-12 9:53 ` Maxim Levitsky
2021-10-15 16:15 ` Sean Christopherson
2021-10-15 16:23 ` Paolo Bonzini [this message]
2021-10-15 16:36 ` Sean Christopherson
2021-10-15 17:50 ` Paolo Bonzini
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