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

[Mikael Pettersson <mikpe@it.uu.se>]

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.

Nice.

 > 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.

Thanks,

/Mikael