linux-kernel - Re: [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes

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Message-ID: <5faa7e49-9eb6-a075-982a-aa7947a5a3d6@redhat.com>
Date:   Fri, 15 Oct 2021 18:23:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] KVM: x86: Fix and cleanup for recent AVIC changes

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knowingly out of date.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