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

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Date:   Tue, 19 May 2020 10:06:25 +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 19/05/20 09:55, Sean Christopherson wrote:
>> Running arbitrary code under the emulator is problematic anyway with
>> CET, since you won't be checking ENDBR markers or updating the state
>> machine.  So perhaps in addition to what you say we should have a mode
>> where, unless unrestricted guest is disabled, the emulator only accepts
>> I/O, MOV and ALU instructions.
>
> Doh, I forgot all about those pesky ENDBR markers.  I think a slimmed down
> emulator makes sense?

Or just slimmed down opcode tables.

> Tangentially related, isn't the whole fastop thing doomed once CET kernel
> support lands?

Why?  You do need to add endbr markers and some of the fastop handlers
won't fit in 8 bytes, but that should be it.

Paolo