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Message-ID: <bc3951cc-2c57-6ce4-6218-2f9bb4ad8196@linux.alibaba.com>
Date:   Wed, 8 Dec 2021 17:34:11 +0800
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Subject: Re: [PATCH 16/15] KVM: X86: Update mmu->pdptrs only when it is
 changed
Content-Language: en-US
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        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>
 <60743c95-f9aa-a7c6-1709-39c70e224321@redhat.com>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
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On 2021/12/8 17:09, Paolo Bonzini wrote:
> 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.
> 


They can be not synced in other side: not available.

If (!kvm_register_is_available(vcpu, VCPU_EXREG_PDPTR))
it will make no sense to compare mmu->pdptrs when EPT is enabled.

Because vmcs might have different copy, it is better to just mark it
dirty in load_pdptrs().

(SVM is OK even with NPT enabled, since vmcb doesn't have a copy)

I haven't investigated enough then and today.  It is quit complicated.

Thanks
Lai

> 
>> 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.