Re: [PATCH 30/34] x86, fpu: allow setting of XSAVE state

[Thomas Gleixner]

On Thu, 3 Dec 2015, Dave Hansen wrote:

> 
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> We want to modify the Protection Key rights inside the kernel, so
> we need to change PKRU's contents.  But, if we do a plain
> 'wrpkru', when we return to userspace we might do an XRSTOR and
> wipe out the kernel's 'wrpkru'.  So, we need to go after PKRU in
> the xsave buffer.
> 
> We do this by:
> 1. Ensuring that we have the XSAVE registers (fpregs) in the
>    kernel FPU buffer (fpstate)
> 2. Looking up the location of a given state in the buffer
> 3. Filling in the stat
> 4. Ensuring that the hardware knows that state is present there
>    (basically that the 'init optimization' is not in place).
> 5. Copying the newly-modified state back to the registers if
>    necessary.
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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