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

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Date:   Wed, 8 Dec 2021 17:34:11 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        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 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.