linux-kernel - Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32

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Message-ID: <763f3b3f-861e-1716-f65f-1b4f256590c5@redhat.com>
Date:   Mon, 7 Aug 2023 08:55:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yang Weijiang <weijiang.yang@...el.com>
Cc:     peterz@...radead.org, john.allen@....com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
        chao.gao@...el.com, binbin.wu@...ux.intel.com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest
 MSR_IA32_XSS

On 8/4/23 20:27, Sean Christopherson wrote:
> I think my preference is to enforce guest CPUID for host accesses to 
> XSS, XFD, XFD_ERR, etc I'm pretty sure I've advocated for the exact 
> opposite in the past, i.e. argued that KVM's ABI is to not enforce 
> ordering between KVM_SET_CPUID2 and KVM_SET_MSR. But this is becoming
> untenable, juggling the dependencies in KVM is complex and is going
> to result in a nasty bug at some point.

Fortunately, you are right now.  Well, almost :) but the important part 
is that indeed the dependencies are too complex.

While host-side accesses must be allowed, they should only allow the 
default value if the CPUID bit is not set.

Paolo