Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

[Andi Kleen <andi@firstfloor.org>]

Suresh Siddha wrote:
> On Mon, May 19, 2008 at 04:52:01PM +0200, Mikael Pettersson wrote:
>>> But we can
>>> use some what similar magic, if the fxsave/fxrstor give away
>>> some of the fields at the end of fxsave image (today it is reserved
>>> and ignored during fxsave/fxrstor) for software use.
>>> We can then use these fields at the end of fpstate, to indicate the presence of
>>> xstate. But this requires some architecture changes like giving
>>> away this space for SW use. We can take this to architects and
>>> see what they think.
>> If the HW doesn't store anything valuable there, we could store
>> SW flags/cookies there on signal delivery, and clear them before
>> fxrstor (unless the HW is known to ignore those fields).
>> But it depends on how forgiving the HW is.
> 
> Ok. CPU folks are planning to make some of the bytes at the end of fxsave
> image, SW usable.

Are they always zeroed in earlier CPUs though?  If not that wouldn't
work 100% reliably because whatever cookie you put in could have been
there before by chance.

I don't see anything in the SDM guaranteeing zeroing.

-Andi