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

[Keir Fraser <keir@xen.org>]

On 02/10/2012 16:27, "Jan Beulich" <JBeulich@suse.com> 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@suse.com>

Ugly. I'd prefer not to bother unless there really is a win we could care
about here.

 -- Keir