linux-kernel - Re: [PATCH 1/4] KVM: X86: Fix tlb flush for tdp in kvm_invalidate

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Message-ID: <945500f6-27e1-ed81-2b7d-c709cb1d4b33@redhat.com>
Date:   Thu, 21 Oct 2021 19:13:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        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 1/4] KVM: X86: Fix tlb flush for tdp in
 kvm_invalidate_pcid()

On 21/10/21 16:52, Sean Christopherson wrote:
>> I think the EPT violation happens*after*  the cr3 write.  So the instruction to be
>> emulated is not "cr3 write".  The emulation will queue fault into guest though,
>> recursive EPT violation happens since the cr3 exceeds maxphyaddr limit.
> Doh, you're correct.  I think my mind wandered into thinking about what would
> happen with PDPTRs and forgot to get back to normal MOV CR3.
> 
> So yeah, the only way to correctly handle this would be to intercept CR3 loads.
> I'm guessing that would have a noticeable impact on guest performance.

Ouch... yeah, allow_smaller_maxphyaddr already has bad performance, but 
intercepting CR3 loads would be another kind of slow.

Paolo

> Paolo, I'll leave this one for you to decide, we have pretty much written off
> allow_smaller_maxphyaddr:-)