Re: [PATCH 4/5] KVM: Fix asm constraints with CONFIG_FRAME_POINTER=n

[Herbert Xu <herbert@gondor.apana.org.au>]

Avi Kivity <avi@qumranet.com> wrote:
> A "g" constraint may place a local variable in an %rsp-relative memory operand.
> but if your assembly changes %rsp, the operand points to the wrong location.
> 
> An "r" constraint fixes that.
> 
> Thanks to Ingo Molnar for neatly bisecting the problem.
> 
> Signed-off-by: Avi Kivity <avi@qumranet.com>
> 
> Index: linux-2.6/drivers/kvm/vmx.c
> ===================================================================
> --- linux-2.6.orig/drivers/kvm/vmx.c
> +++ linux-2.6/drivers/kvm/vmx.c
> @@ -1825,7 +1825,7 @@ again:
> #endif
>                "setbe %0 \n\t"
>                "popf \n\t"
> -             : "=g" (fail)
> +             : "=r" (fail)
>              : "r"(vcpu->launched), "d"((unsigned long)HOST_RSP),
>                "c"(vcpu),
>                [rax]"i"(offsetof(struct kvm_vcpu, regs[VCPU_REGS_RAX])),

We need the following fix for 2.6.20.

[KVM] vmx: Fix register constraint in launch code

Both "=r" and "=g" breaks my build on i386:

$ make
  CC [M]  drivers/kvm/vmx.o
{standard input}: Assembler messages:
{standard input}:3318: Error: bad register name `%sil'
make[1]: *** [drivers/kvm/vmx.o] Error 1
make: *** [_module_drivers/kvm] Error 2

The reason is that setbe requires an 8-bit register but "=r" does not
constrain the target register to be one that has an 8-bit version on
i386.

According to

	http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10153

the correct constraint is "=q".

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
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--
diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
index ce219e3..0aa2659 100644
--- a/drivers/kvm/vmx.c
+++ b/drivers/kvm/vmx.c
@@ -1824,7 +1824,7 @@ again:
 #endif
 		"setbe %0 \n\t"
 		"popf \n\t"
-	      : "=g" (fail)
+	      : "=q" (fail)
 	      : "r"(vcpu->launched), "d"((unsigned long)HOST_RSP),
 		"c"(vcpu),
 		[rax]"i"(offsetof(struct kvm_vcpu, regs[VCPU_REGS_RAX])),