linux-kernel - Re: [PATCH 16/15] KVM: X86: Update mmu->pdptrs only when it is changed

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Message-ID: <60743c95-f9aa-a7c6-1709-39c70e224321@redhat.com>
Date:   Wed, 8 Dec 2021 10:09:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 16/15] KVM: X86: Update mmu->pdptrs only when it is
 changed

On 12/8/21 00:43, Sean Christopherson wrote:
> what guarantees the that PDPTRs in the VMCS are sync'd with
> mmu->pdptrs?  I'm not saying they aren't, I just want the changelog
> to prove that they are.

If they aren't synced you should *already* have dirty VCPU_EXREG_PDPTR 
and pending KVM_REQ_LOAD_MMU_PGD, shouldn't you?  As long as the caching 
invariants are respected, this patch is fairly safe, and if they aren't 
there are plenty of preexisting bugs anyway.

Paolo

> The next patch does add a fairly heavy unload of the current root for
> !TDP, but that's a bug fix and should be ordered before any
> optimizations anyways.