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

["H. Peter Anvin" <hpa@zytor.com>]

Mikael Pettersson wrote:
> 
> My problem with the OSXAVE flag is that it's a very indirect way of
> communicating the layout of sigframes and sigcontexts. These structures
> should, if at all possible, be self-describing. A single flag bit in
> the sigcontext could handle both structures (since a sigframe always
> includes a sigcontext).
> 

It's also wrong, since OSXSAVE indicates that the CPU can do it, not 
that the kernel can.

> 
>>> struct _fpstate has a 'magic' field which distinguishes x87-only
>>> from x87+FXSR structs. Could that field also be used to indicate XSAVE?
>> I don't think we can use the existing 'magic' field.
> 
> Hmm, right now it seems this field has a de-facto ABI of being
> either 0xffff (plain) or 0x0000 (fxsr). Using other values would
> confuse at least one application I know of. Sad.
> 

Well, arguably it is the right thing to use since we're talking about a 
new format.  The difference is that the new format *does* extend 
backwards to match the old format.

>> 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.

All we need is a single field -- a single byte -- reserved indefinitely 
for software use.  Existing FXSAVE kernels will have set it to zero.

There might be fields the existing FXSAVE format which can be equally 
abused, even.  I will do some looking.

	-hpa