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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.12.2019 21:51, Andrew Cooper wrote:
> 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".

The AMD PM has "Instead, they are treated as null prefixes." This is
what I've taken to use here. I'm happy to take whatever other
sensible wording you like better (including "ignored"). But I'd like
you to explicitly clarify that you're not okay with me using a term
from vendor documentation here.

> 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.

Objdump getting it wrong is no surprise at all to me (which is one
of the reasons why I prefer to use my own disassembler wherever
possible). Yet without you spelling out what specific anomalies
you've observed (or what weirdness there is with single stepping)
I won't know whether I may want to make an attempt at fixing
objdump. Nor can I see what this comment's implication is on the
patch here, i.e. what changes you mean me to make.

Jan

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