Re: [tip:x86/urgent] x86/fpu: Set the xcomp_bv when we fake up a XSAVES area

[Dave Hansen]

>> The fix I am proposing is...
>>
>> 	state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
>> 				       xfeatures_mask;
> 
> Actually I thought about this change before I made this patch, but I don't this
> is the right fix. It is always error prone to init the xcomp_bv to all the
> supported feature. In case like copyin_to_xsaves(), it is possible that the
> features which should be set in xcomp_bv do not equal to all the supported
> features. Please see the following codes in copyin_to_xsaves():
> 	/*
> 	 * The state that came in from userspace was user-state only.
> 	 * Mask all the user states out of 'xfeatures':
> 	 */
> 	xsave->header.xfeatures &= XFEATURE_MASK_SUPERVISOR;
> 
> 	/*
> 	 * Add back in the features that came in from userspace:
> 	 */
> 	xsave->header.xfeatures |= xfeatures;

Hi Kevin,

I think you may be confusing 'xfeatures' with 'xcomp_bv'.  xfeatures
tells you what features are present in the buffer while xcomp_bv tells
you what *format* the buffer is in.

Userspace never dictates the *format* of the kernel buffer, only the
contents of each state.  So, it makes sense that the copyin code would
not (and should not) modify xcomp_bv.

We ensure that xcomp_bv has all supported states set all the time, or
we're *supposed* to.