Re: [Xen-devel] Ping#2: [PATCH] x86emul: keep compiler from using {x, y, z}mm registers itself

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 11/21/2017 01:29 PM, Andrew Cooper wrote:
> On 21/11/17 13:26, Jan Beulich wrote:
> > > > > On 06.11.17 at 16:04, <george.dunlap@xxxxxxxxxx> wrote:
> > > On 11/06/2017 11:59 AM, Jan Beulich wrote:
> > > > > > > On 16.10.17 at 14:42,  wrote:
> > > > > > > > On 16.10.17 at 14:37, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > > On 16/10/17 13:32, Jan Beulich wrote:
> > > > > > > Since the emulator acts on the live hardware registers, we need to
> > > > > > > prevent the compiler from using them e.g. for inlined memcpy() /
> > > > > > > memset() (as gcc7 does). We can't, however, set this from the command
> > > > > > > line, as otherwise the 64-bit build would face issues with functions
> > > > > > > returning floating point values and being declared in standard headers.
> > > > > > >
> > > > > > > As the pragma isn't available prior to gcc6, we need to invoke it
> > > > > > > conditionally. Luckily up to gcc6 we haven't seen generated code access
> > > > > > > SIMD registers beyond what our asm()s do.
> > > > > > >
> > > > > > > Reported-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> > > > > > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > > > > > ---
> > > > > > > While this doesn't affect core functionality, I think it would still be
> > > > > > > nice for it to be allowed in for 4.10.
> > > > > > Agreed.
> > > > > >
> > > > > > Has this been tested with Clang?
> > > > > Sorry, no - still haven't got around to set up a suitable Clang
> > > > > locally.
> > > > >
> > > > > >   It stands a good chance of being
> > > > > > compatible, but we may need an && !defined(__clang__) included.
> > > > > Should non-gcc silently ignore "#pragma GCC ..." it doesn't
> > > > > recognize, or not define __GNUC__ in the first place if it isn't
> > > > > sufficiently compatible? I.e. if anything I'd expect we need
> > > > > "#elif defined(__clang__)" to achieve the same for Clang by
> > > > > some different pragma (if such exists).
> > > > Not having received any reply so far, I'm wondering whether
> > > > being able to build the test harness with clang is more
> > > > important than for it to work correctly when built with gcc. I
> > > > can't predict when I would get around to set up a suitable
> > > > clang on my dev systems.
> > > I agree with the argument you make above.  On the unlikely chance
> > > there's a problem Travis should catch it, and someone who actually has a
> > > clang setup can help sort it out.
> > I'm still lacking an ack, before it being sensible to check with Julien
> > whether this is still fine to go in at this late stage.
I would be happy with that.

Release-acked-by: Julien Grall <julien.grall@xxxxxxxxxx>

Cheers,

--
Julien Grall

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