Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in xsave_user/xrestore_user

Quentin Casasnovas
2015-03-17T09:59:28Z

On Tue, Mar 17, 2015 at 10:47:50AM +0100, Borislav Petkov wrote:
> On Mon, Mar 16, 2015 at 11:37:44PM +0100, Quentin Casasnovas wrote:
>
> ...
>
> > __user_insn("btl [var2], %0 \n\t",
> > , /* no outputs, no need for dummy arg */
> > SINGLE_ARG("r" (var1), [var2] "r" (var2)), /* two inputs */
> > "cc");
>
> So this becomes pretty unreadable IMO. And we shouldn't go nuts with
> optimizing this and sacrifice readability a lot.
>
> TBH, I'd much prefer:
>
> if (static_cpu_has_safe(X86_FEATURE_XSAVEOPT)) {
> check_insn(XSAVEOPT, ...);
^
> return;
> }
>
> if (static_cpu_has_safe(X86_FEATURE_XSAVES)) {
> check_insn(XSAVES);
> return;
> }
>
> check_insn(XSAVE, ...)
>
> which is pretty clear.
>

Fair point, but AFAIUI we can't do check_insn(XSAVES) alone as of today,
and the "..." in your "check_isns(XSAVEOPT, ...)" code above would still
need to contain the outputs operands.

My suggestion was to rework (check|user)_insn() so it can accept zero to N
inputs, outputs or clobbers to make it generic enough so the snippet of
code you've written becomes valid, and maybe move those macro where they
can be used for other sub-systems?

Am I missing something?

Quentin