Re: [PATCH v5 12/13] x86/xsaves: Fix fpstate_init() for XSAVES

[Yu-cheng Yu]

On Mon, May 09, 2016 at 04:41:31PM -0700, Dave Hansen wrote:
> On 05/09/2016 01:46 PM, Yu-cheng Yu wrote:
> > When XSAVES is used, xsave.header.xcomp_bv[63] must be set.
> 
> So, that's not strictly correct.  XSAVES can write to a completely empty
> (0'd) memory buffer.  It's only XRSTORS that _needs_ bit 63 set.  The
> instruction reference is pretty clear on this point.

You mean the comments?  I will change it to XRSTORS.

> 
> Oh, and if you decided to do this for some reason, please have mercy and
> go make a macro for 1<<63.
> 
> Also, I don't think the kernel ever checks for this bit.  So are we
> really calling XRSTORS on otherwise uninitialized xsave buffers?

Right now if we do fpstate_init(), without this patch, it will 
trigger a warning from copy_kernel_to_fxregs() when the task is
scheduled.