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

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

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