linux-kernel - Re: [PATCH 1/7] KVM: x86: Retry page fault if MMU reload is pending and root has no sp

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Date:   Fri, 10 Dec 2021 17:13:29 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.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, Maxim Levitsky <mlevitsk@...hat.com>,
        Ben Gardon <bgardon@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH 1/7] KVM: x86: Retry page fault if MMU reload is pending
 and root has no sp

On 12/10/21 17:01, Sean Christopherson wrote:
>> KVM_REQ_MMU_RELOAD is raised after kvm->arch.mmu_valid_gen is fixed (of
>> course, otherwise the other CPU might just not see any obsoleted page
>> from the legacy MMU), therefore any check on KVM_REQ_MMU_RELOAD is just
>> advisory.
> 
> I disagree.  IMO, KVM should not be installing SPTEs into obsolete shadow pages,
> which is what continuing on allows.  I don't _think_ it's problematic, but I do
> think it's wrong.
>
> [...] Eh, for all intents and purposes, KVM_REQ_MMU_RELOAD very much says
> special roots are obsolete.  The root will be unloaded, i.e. will no
> longer be used, i.e. is obsolete.

I understand that---but it takes some unspoken details to understand 
that.  In particular that both kvm_reload_remote_mmus and 
is_page_fault_stale are called under mmu_lock write-lock, and that 
there's no unlock between updating mmu_valid_gen and calling 
kvm_reload_remote_mmus.

(This also suggests, for the other six patches, keeping 
kvm_reload_remote_mmus and just moving it to arch/x86/kvm/mmu/mmu.c, 
with an assertion that the MMU lock is held for write).

But since we have a way forward for having no special roots to worry 
about, it seems an unnecessary overload for 1) a patch that will last 
one or two releasees at most 2) a case that has been handled in the 
inefficient way forever.

Paolo

> The other way to check for an invalid special root would be to treat
> it as obsolete if any of its children in entries 0-3 are present and
> obsolete.  That would be more precise, but it provides no benefit
> given KVM's current implementation.
> 
> I'm not completely opposed to doing nothing, but I do think it's
> silly to continue on knowing that the work done by the page fault is
> all but gauranteed to be useless.
>