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Re: [Xen-devel] [PATCH] x86emul: correct segment override decode for 64-bit mode


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 11 Dec 2019 20:51:56 +0000
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  • Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Wed, 11 Dec 2019 20:52:07 +0000
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On 11/12/2019 09:27, Jan Beulich wrote:
> The legacy / compatibility mode ES, CS, SS, and DS overrides are null
> prefixes in 64-bit mode, i.e. they in particular don't cancel an
> earlier FS or GS one.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

null is a very overloaded term.  What you mean here is simply "ignored".

In attempting to confirm/test this, I've found yet another curiosity
with instruction length calculations when reordering a rex prefix and
legacy prefix.  Objdump gets it wrong, but the instruction boundaries
according to singlestep are weird.

~Andrew

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