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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 21 Oct 2021 10:32:49 -0700
Message-ID: <CALMp9eSAYYL2T_H5b3Kv+OE2KgDz_iC32yQfpiqhwspRUezQ2w@mail.gmail.com>
Subject: Re: [PATCH 1/4] KVM: X86: Fix tlb flush for tdp in kvm_invalidate_pcid()
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.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>
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On Thu, Oct 21, 2021 at 10:13 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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.

Can we kill it? It's only half-baked as it is. Or are we committed to it now?