Re: [Xen-devel] [PATCH 3/3] x86: save/restore only partial register state where possible

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Keir Fraser <keir@xxxxxxx> 10/02/12 7:02 PM >>>
>On 02/10/2012 16:27, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>
>> ... and make restore conditional not only upon having saved the state,
>> but also upon whether saved state was actually modified (and register
>> values are known to have been preserved).
>> 
>> Note that RBP is unconditionally considered a volatile register (i.e.
>> irrespective of CONFIG_FRAME_POINTER), since the RBP handling would
>> become overly complicated due to the need to save/restore it on the
>> compat mode hypercall path [6th argument].
>>
>> Note further that for compat mode code paths, saving/restoring R8...R15
>> is entirely unnecessary - we don't allow those guests to enter 64-bit
>> mode, and hence they have no way of seeing these registers' contents
>> (and there consequently also is no information leak, except if the
>> context saving domctl would be considered such).
>>
>> Finally, note that this may not properly deal with gdbstub's needs, yet
>> (but if so, I can't really suggest adjustments, as I don't know that
>> code).
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
>Ugly. I'd prefer not to bother unless there really is a win we could care
>about here.

Without this patch, patch 1 doesn't make a lot of sense either (and patch 2 then
is merely cleanup).

Jan


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