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

[H. Peter Anvin]

Suresh Siddha wrote:
> > >
> > It's also wrong, since OSXSAVE indicates that the CPU can do it, not 
> > that the kernel can.
>
> OSXSAVE indicates the OS support and XSAVE indicates the cpu support.

Sorry, brainfart.  Don't post so early in the morning.

> > 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.
>
> All the reserved fields at the end of fxsave format are zeroed and
> presented as such to the user. If HW makes some of these fields SW available,
> then we can use those (will check). If there is any scope with the
> existing format it self, that will be much better.

I was thinking about what we'd really like earlier, and given a clean 
slate I'd like to see a structure looking like:

struct state_ptrs {
	size_t len;
	struct state_foo *foo;
	struct state_bar *bar;
	...
};

... where len is sizeof(struct state_ptrs).  This is not merely 
extensible, but it's easy for userspace to massage it into whatever 
format -- longer or shorter -- that it happens to know about, and it 
gives a natural way for the kernel to communicate "none of this state" 
by feeding a NULL pointer.  So pretty much we're looking for a way to 
backwards-compatible way to stash a pointer to this structure, I figure.

	-hpa
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