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[Xen-devel] Re: [PATCH] fxsave/fxrstor adjustments

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] fxsave/fxrstor adjustments
From: Andi Kleen <ak@xxxxxxx>
Date: 25 Apr 2006 02:56:17 +0200
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, jbeulich@xxxxxxxxxx
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Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> writes:

> On 24 Apr 2006, at 16:31, Jan Beulich wrote:
> > This patch addresses CVE-2006-1056 (information leak from
> > fxsave/fxrstor on AMD CPUs) and also adjusts 64-bit handling
> > so that full 64-bit RIP/RDP values get saved/restored. More
> > fine-grained handling may be needed if 32-bit processes are
> > expected to properly see their selectors (native Linux doesn't
> > currently do that either, but there is a patch to adjust
> > it there).
> Why does this patch (and the one in Linux use 'emms' in the
> fxsave path rather than 'ffree st(7)' which is what AMD recommends in
> their published advisory? Is the former faster?

On K7/K8 emms and ffree st(7) are the same performance. On P4 ffree is
much faster. The Linux 2.6 patch uses emms because it patches the code
in only on K7/K8. For the Xen patch that's ok too although it checks,
not patches. In Linux 2.4 where the code is executed unconditionally
ffree is used.

An earlier version of the AMD workaround used emms always until
the P4 emms performance issue was discovered.


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