linux-kernel - Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

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Date:   Mon, 18 May 2020 19:37:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

On 18/05/20 18:07, Sean Christopherson wrote:
> On Fri, May 15, 2020 at 12:19:19PM -0400, Paolo Bonzini wrote:
>> Instructions starting with 0f18 up to 0f1f are reserved nops, except those
>> that were assigned to MPX.
> Well, they're probably reserved NOPs again :-D.

So are you suggesting adding them back to the list as well?

>> These include the endbr markers used by CET.
> And RDSPP.  Wouldn't it make sense to treat RDSPP as a #UD even though it's
> a NOP if CET is disabled?  The logic being that a sane guest will execute
> RDSSP iff CET is enabled, and in that case it'd be better to inject a #UD
> than to silently break the guest.

We cannot assume that guests will bother checking CPUID before invoking
RDSPP.  This is especially true userspace, which needs to check if CET
is enable for itself and can only use RDSPP to do so.

Paolo