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

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Message-ID: <b148def5-9d34-bfa7-db6e-afaf11728639@redhat.com>
Date:   Fri, 15 Oct 2021 19:50:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        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 18:36, Sean Christopherson wrote:
>> 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 out of date.
> If that's the argument, then kvm_faultin_page() should explicitly check for a
> pending KVM_REQ_APICV_UPDATE, because I would then argue that contuining on when
> KVM_knows_  its new SPTE will either get zapped (page fault wins the race) or
> will get rejected (kvm_zap_gfn_range() wins the race) is just as wrong.  The SPTE
> _cannot_  be used even if the page fault wins the race, becuase all vCPUs need to
> process KVM_REQ_APICV_UPDATE and thus will be blocked until the initiating vCPU
> zaps the range and drops the APICv lock.

Right, that was my counter-argument - no need to check for the request 
because the request "synchronizes" with the actual use of the PTE, via 
kvm_make_all_cpus_request + kvm_zap_gfn_range.

> And I personally do_not_  want to add a check for the request because it implies
> the check is sufficient, which it is not, because the page fault doesn't yet hold
> mmu_lock.

Of course, that would be even worse.

> Since all answers are some form of wrong, IMO we should at least be coherent with
> respect to the original page fault.

Okay, you win if you send a patch with a comment. :)

Paolo