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



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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