[ovs-discuss] [PATCH net-next] fast_hash: clobber registers correctly for inline function use
Hannes Frederic Sowa
hannes at stressinduktion.org
Fri Nov 14 20:04:11 UTC 2014
On Fr, 2014-11-14 at 13:38 -0500, David Miller wrote:
> From: Hannes Frederic Sowa <hannes at stressinduktion.org>
> Date: Fri, 14 Nov 2014 16:46:18 +0100
> > I would still like to see the current proposed fix getting applied and
> > we can do this on-top. The inline call after this patch reassembles a
> > direct function call, so besides the long list of clobbers, it should
> > still be pretty fast.
> I would rather revert the change entirely until it is implemented
> Also, I am strongly of the opinion that this is a mis-use of the
> alternative call interface. It was never intended to be used for
> things that can make real function calls.
I tend to disagree. Grepping e.g. shows
ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from),
"1" (to), "2" (from), "3" (len)
: "memory", "rcx", "r8", "r9", "r10", "r11");
(it has a few less clobbers because it has more output operands)
I just tried to come up with some macros which lets you abstract away
the clobber list, but in the end it somehow has to look exactly like
that. The double-colon syntax also makes it difficult to come up with
something that let's us use varargs for that.
> You can add a million clobbers, or a trampoline, it's still using a
> facility in a manner for which it was not designed.
The full clobber list for a function call which would always clear
registers like we would have in a normal non-inlined function call would
look like this:
#define FUNC_CLOBBER LIST "memory", "cc", "rax", "rdi", "rsi", "rdx", "rcx", "r8", "r9", "r10", "r11"
(reference in arch/x86/include/asm/calling.h).
> This means a new interface with a new name and with capabilities
> explicitly supporting this case are in order.
It try to implicitly embed the clobber list, would something like that
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