Messages in this thread | | | Date | Tue, 20 May 2008 10:58:20 +0200 | From | Mikael Pettersson <> | Subject | Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions |
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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
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