linux-kernel - Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO

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Message-ID: <3b7afdfc-d308-7645-2105-6bb339188aa6@redhat.com>
Date:   Fri, 18 Aug 2017 14:35:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast
 MMIO

On 18/08/2017 13:57, David Hildenbrand wrote:
> What about a hack:
> 
> 1. clear instruction length when entering
> 2. check if instruction length is set when trying to forward the RIP
> 2a. if set, use it
> 2b. if not set, compute it

It's undefined, so we don't know that the instruction length remains
zero (also, on older processors and possibly some nested setups the
field is read-only).

Testing the hypervisor bit is the first line of action.

Paolo

> this at least should give full speedup in existing setups. Not 99%
> architecturally correct but might just work.
>