Re: [PATCH] x86/xsave: Robustify and merge macros

From: Quentin Casasnovas
Date: Thu Apr 02 2015 - 12:32:04 EST

On Thu, Apr 02, 2015 at 06:12:59PM +0200, Borislav Petkov wrote:
> On Thu, Apr 02, 2015 at 05:52:10PM +0200, Quentin Casasnovas wrote:
> > FWIW I think this looks much nicer! I have a couple of comments though,
> > apologies in advance if they aren't relevant :)
> No worries, I very much appreciate the looking at. :)


> > I thought the SYSTEM_BOOTING checks were present to make sure we call these
> > functions only when the alternative instructions had *not* been applied
> > (i.e. when SYSTEM_BOOTING). We could have added the opposite checks in
> > xsave_state()/xrstor_state() to make sure the alternative instructions are
> > applied when these are called (i.e. when !SYSTEM_BOOTING).
> Well, I think this was a clumsy way to say that we shouldn't be using
> the _booting() variants when the system isn't booting anymore:
> - WARN_ON(system_state != SYSTEM_BOOTING);
> - if (boot_cpu_has(X86_FEATURE_XSAVES))
> - asm volatile("1:"XSAVES"\n\t"
> - "2:\n\t"
> - xstate_fault
> - : "D" (fx), "m" (*fx), "a" (lmask), "d" (hmask)
> - : "memory");
> else
> - asm volatile("1:"XSAVE"\n\t"
> - "2:\n\t"
> - xstate_fault
> - : "D" (fx), "m" (*fx), "a" (lmask), "d" (hmask)
> - : "memory");
> WRT alternatives, the code didn't have any alternatives invocations
> there - it is just a cluttered way of saying:
> if (CPU has XSAVES support)
> else

So I'm not saying the function is using alternative, just that if the
alternative _are_ applied, then we do not want to use the *_booting()
variants (likely for performance reasons), hence the WARN_ON().

So IMO it does not hurt to keep it here, with maybe renaming it something
like the following so it is obvious why it's there:

/* Use the non _booting() variants if the alternatives are applied. */

I would personnaly add it to the non _booting() variants as well to make
sure the alternative instructions _are_ applied, since otherwise that would
probably cause random failures to restore the xsaveopt/xsaves context
previously saved. Obviously very paranoid check anyway so if you still
want to drop it then fine :)

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