Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <60743c95-f9aa-a7c6-1709-39c70e224321@redhat.com>
Date:   Wed, 8 Dec 2021 10:09:30 +0100
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Subject: Re: [PATCH 16/15] KVM: X86: Update mmu->pdptrs only when it is
 changed
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To:     Sean Christopherson <seanjc@google.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
References: <20211108124407.12187-1-jiangshanlai@gmail.com>
 <20211111144527.88852-1-jiangshanlai@gmail.com> <Ya/xsx1pcB0Pq/Pm@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <Ya/xsx1pcB0Pq/Pm@google.com>
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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.