On 11/14/2012 11:45 AM, Blower, Melanie wrote:
> [1.] gcc -O0 assembly arch/x86/kvm/emulate.c gets compilation failure --
> incorrect register restrictions
> [2.] Full description of the problem/report:
> I'm trying to compile this file at -O0, but gcc chokes in register allocation
> at the inline assembly.
>
> In the ordinary Linux build, this file compiles with gcc at -O2, without
> compilation errors.
>
> At -O0, gcc chokes with this message:
> gcc -w -c ./emulateE.c // (using preprocessed file)
> ./emulateE.c: In function `em_mul_ex':
> ./emulateE.c:1918:5: error: can't find a register in class `AREG' while
> reloading `asm'
> ./emulateE.c:1918:5: error: `asm' operand has impossible constraints
>
> Explanation:
> The file contains an inline asm of a kind:
>
> __asm__ __volatile__ ( " ..... " :
>
> "=m" ((ctxt)->eflags), "=&r" (_tmp), "+a" (*rax), "+d" (*rdx), "+qm"(ex) :
> "i" (11), "m" ((ctxt)->src . val), "a" (*rax), "d" (*rdx));
>
> Note that "+a" in inputs already means that eax is the return value. An then
> "a" is used as an output constraint too.
>
Hi Melanie,
Can you test the attached patch?
-hpa
>From f8d8e2842ca05fd89788e35c087f02c6159b023a Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <[email protected]>
Date: Wed, 21 Nov 2012 13:29:20 -0800
Subject: [PATCH] x86, kvm: Remove incorrect redundant assembly constraint
In __emulate_1op_rax_rdx, we use "+a" and "+d" which are input/output
constraints, and *then* use "a" and "d" as input constraints. This is
incorrect, but happens to work on some versions of gcc.
However, it breaks gcc with -O0 and icc, and may break on future
versions of gcc.
Reported-by: Melanie Blower <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>
Link: http://lkml.kernel.org/r/b3584e72cfebed439a3eca9bce67a4ef1b17a...@fmsmsx107.amr.corp.intel.com
---
arch/x86/kvm/emulate.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 39171cb..bba39bf 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -426,8 +426,7 @@ static void invalidate_registers(struct x86_emulate_ctxt *ctxt)
_ASM_EXTABLE(1b, 3b) \
: "=m" ((ctxt)->eflags), "=&r" (_tmp), \
"+a" (*rax), "+d" (*rdx), "+qm"(_ex) \
- : "i" (EFLAGS_MASK), "m" ((ctxt)->src.val), \
- "a" (*rax), "d" (*rdx)); \
+ : "i" (EFLAGS_MASK), "m" ((ctxt)->src.val)); \
} while (0)
/* instruction has only one source operand, destination is implicit (e.g. mul, div, imul, idiv) */
--
1.7.11.7
