linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Date:	Mon, 19 May 2008 10:45:37 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Mikael Pettersson <mikpe@...uu.se>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, andi@...stfloor.org, roland@...hat.com,
	drepper@...hat.com, Hongjiu.lu@...el.com,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com,
	rmk+lkml@....linux.org.uk, dan@...ian.org, asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

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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