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

From: Mikael Pettersson
Date: Tue May 20 2008 - 04:59:56 EST

Suresh Siddha writes:
> 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.


> We can use some of these fields, to represent the extended state
> presence with a cookie, save area size, mask of the state
> stored. If needed, we can include the start address of the fpstate pointer
> (also as part of the cookie), so that we can detect the situation,
> where apps are just memcopying sizeof(struct _fpstate) from the fpstate
> pointer (but not aware of the extended state).

I use a similar technique to detect user-space mangling
of ucontexts on Solaris.

> we don't need any ucontext_t extensions any more and just
> use the fpstate pointer to indicate the extended state aswell, right?

Yes, the old magic distinguishes x87-only from x87+fxsr, the new magic
distinguishes fxsr from xsave.

> In addition, we need to make sure that for 32bit non-rt sigframes, we
> don't modify the extramask[] offset.


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