linux-kernel - Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

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Date:   Tue, 12 Jan 2021 23:03:58 -0600
From:   Wei Huang <wei.huang2@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, joro@...tes.org,
        bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, bsd@...hat.com,
        dgilbert@...hat.com, mlevitsk@...hat.com
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions



On 1/12/21 11:59 AM, Sean Christopherson wrote:
> On Tue, Jan 12, 2021, Sean Christopherson wrote:
>> On Tue, Jan 12, 2021, Wei Huang wrote:
>>> From: Bandan Das <bsd@...hat.com>
>>>
>>> While running VM related instructions (VMRUN/VMSAVE/VMLOAD), some AMD
>>> CPUs check EAX against reserved memory regions (e.g. SMM memory on host)
>>> before checking VMCB's instruction intercept.
>>
>> It would be very helpful to list exactly which CPUs are/aren't affected, even if
>> that just means stating something like "all CPUs before XYZ".  Given patch 2/2,
>> I assume it's all CPUs without the new CPUID flag?

This behavior was dated back to fairly old CPUs. It is fair to assume 
that _most_ CPUs without this CPUID bit can demonstrate such behavior.

> 
> Ah, despite calling this an 'errata', the bad behavior is explicitly documented
> in the APM, i.e. it's an architecture bug, not a silicon bug.
> 
> Can you reword the changelog to make it clear that the premature #GP is the
> correct architectural behavior for CPUs without the new CPUID flag?

Sure, will do in the next version.

>