Re: [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer address finder to support dynamic states

Borislav Petkov
2021-08-18T11:32:55Z

On Fri, Jul 30, 2021 at 07:59:41AM -0700, Chang S. Bae wrote:
> __raw_xsave_addr() returns the requested component's pointer in an XSTATE
> buffer, by simply looking up the offset table. The offset used to be fixed,
> but, with dynamic user states, it becomes variable.
>
> get_xstate_size() has a routine to find an offset at runtime. Refactor to
> use it for the address finder.
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Reviewed-by: Len Brown <len.brown@intel.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
> Changes from v5:
> * Updated for future proofed __raw_xsave_addr().
>
> Changes from v3:
> * Added the function description in the kernel-doc style. (Borislav Petkov)
> * Removed 'no functional change' in the changelog. (Borislav Petkov)
> ---
> arch/x86/kernel/fpu/xstate.c | 78 ++++++++++++++++++++++++------------
> 1 file changed, 53 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 26f6d5e0f1ed..98ab10e4da3b 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -182,6 +182,38 @@ static bool xfeature_is_supervisor(int xfeature_nr)
> return ecx & 1;
> }
>
> +/**
> + * get_xstate_comp_offset - Find the feature's offset in the compacted
> + * format.
> + * @mask: This bitmap tells which components reserved in the format.

There's that "reserved" confusion thing. Rewrite pls.

> + * @feature_nr: The feature number
> + *
> + * Returns: The offset value
> + */
> +static unsigned int get_xstate_comp_offset(u64 mask, int feature_nr)
> +{
> + u64 xmask = BIT_ULL(feature_nr + 1) - 1;
> + unsigned int next_offset, offset = 0;
> + int i;
> +
> + if ((xfeatures_mask_all & xmask) == (mask & xmask))
> + return xstate_comp_offsets[feature_nr];
> +
> + /*
> + * With the given mask, no relevant size is found. Calculate it by
> + * summing up each state size.
> + */
> + for (next_offset = FXSAVE_SIZE + XSAVE_HDR_SIZE, i = FIRST_EXTENDED_XFEATURE;
> + i <= feature_nr; i++) {
> + if (!(mask & BIT_ULL(i)))
> + continue;
> +
> + offset = xstate_aligns[i] ? ALIGN(next_offset, 64) : next_offset;
> + next_offset += xstate_sizes[i];

Why is this more complex than it has to be?

IOW, why can't you simply do:

offset = FXSAVE_SIZE + XSAVE_HDR_SIZE;
for (i = FIRST_EXTENDED_XFEATURE; i <= feature_nr; i++) {
if (!(mask & BIT_ULL(i)))
continue;

if (xstate_aligns[i])
offset = ALIGN(offset, 64);

offset += xstate_sizes[i];
}
return offset;

like it was before?

--
Regards/Gruss,
Boris.