linux-kernel - Re: [PATCH 2/4] kvm, vmx: move register clearing out of assembly path

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Date:   Fri, 26 Oct 2018 09:30:17 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Julian Stecklina <jsteckli@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, js@...en8.de,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] kvm, vmx: move register clearing out of assembly path

On Fri, Oct 26, 2018 at 8:46 AM, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:

> Since clearing the GPRs exists to mitigate speculation junk, I think
> we should keep the explicit XOR zeroing instead of deferring to the
> compiler.  Explicit XORs will ensure the resulting assembly is the
> same regardless of compiler, version, target arch, etc..., whereas the
> compiler could theoretically use different zeroing methods[1], e.g. on
> my system it generates "mov r32,r32" for EBX, ESI and EDI (loading
> from EAX after EAX is zeroed).
>
> And FWIW, I find the original code to be more readable since all GRPs
> are zeroed with the same method.

I concur. I really do prefer the explicit xors to the input arguments.