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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 23 Jun 2021 10:00:11 -0700
Message-ID: <CALMp9eSpEJrr6mNoLcGgV8Pa2abQUkPA1uwNBMJZWexBArB3gg@mail.gmail.com>
Subject: Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2}
 after KVM_RUN is broken
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
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On Wed, Jun 23, 2021 at 7:16 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 22/06/21 19:56, Sean Christopherson wrote:
> > +     /*
> > +      * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> > +      * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> > +      * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> > +      * faults due to reusing SPs/SPTEs.  Alert userspace, but otherwise
> > +      * sweep the problem under the rug.
> > +      *
> > +      * KVM's horrific CPUID ABI makes the problem all but impossible to
> > +      * solve, as correctly handling multiple vCPU models (with respect to
> > +      * paging and physical address properties) in a single VM would require
> > +      * tracking all relevant CPUID information in kvm_mmu_page_role.  That
> > +      * is very undesirable as it would double the memory requirements for
> > +      * gfn_track (see struct kvm_mmu_page_role comments), and in practice
> > +      * no sane VMM mucks with the core vCPU model on the fly.
> > +      */
> > +     if (vcpu->arch.last_vmentry_cpu != -1)
> > +             pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n");
>
> Let's make this even stronger and promise to break it in 5.16.
>
> Paolo

Doesn't this fall squarely into kvm's philosophy of "we should let
userspace shoot itself in the foot wherever possible"? I thought we
only stepped in when host stability was an issue.

I'm actually delighted if this is a sign that we're rethinking that
philosophy. I'd just like to hear someone say it.