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

[Dave Hansen <dave.hansen@linux.intel.com>]

On 01/23/2017 08:55 AM, Yu-cheng Yu wrote:
> On Mon, Jan 23, 2017 at 07:36:20AM -0800, Dave Hansen wrote:
>> The kernel xsave buffer should *ALWAYS* have the
>> XCOMP_BV_COMPACTED_FORMAT bit set.  It should have been set before the
>> copyin and it should be set when it's finished.
>>
>> The best fix here would be not to paper over the issue in the copy
>> function but find where it got clobbered, or where some initialization
>> code failed to set it.
> 
> Someone else reported different issues from the same bug and a different
> patch was just tested OK this morning.  I think that adding xfeatures bits
> to xcomp_bv should have been done in fpstate_init().

Right.  So where did it get cleared out?

> Also, in copy_init_fpstate_to_fpregs(), we do:
> 
> 	copy_kernel_to_xregs(&init_fpstate.xsave, -1).
> 
> That (-1) could mean (0) because the parameters are declared as:
> 
> 	copy_kernel_to_xregs(struct xregs_state *, u64)

I'm not sure what you're saying.  -1 just means "all 1's" when cast to
an unsigned type.  This shouldn't case any problems.