Re: [PATCH] arm64: Work around broken GCC 4.9 handling of "S" constraint

[Catalin Marinas]

On Thu, 17 Dec 2020 11:11:35 +, Marc Zyngier wrote:
> GCC 4.9 seems to have a problem with the "S" asm constraint
> when the symbol lives in the same compilation unit, and pretends
> the constraint is impossible:
> 
> $ cat x.c
> void *foo(void)
> {
>   static int x;
>   int *addr;
>   asm("adrp %0, %1" : "=r" (addr) : "S" ());
>   return addr;
> }
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: Work around broken GCC 4.9 handling of "S" constraint
  https://git.kernel.org/arm64/c/9fd339a45be5

-- 
Catalin

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Re: [PATCH] arm64: Work around broken GCC 4.9 handling of "S" constraint

[Ard Biesheuvel]

On Thu, 17 Dec 2020 at 12:11, Marc Zyngier  wrote:
>
> GCC 4.9 seems to have a problem with the "S" asm constraint
> when the symbol lives in the same compilation unit, and pretends
> the constraint is impossible:
>
> $ cat x.c
> void *foo(void)
> {
> static int x;
> int *addr;
> asm("adrp %0, %1" : "=r" (addr) : "S" ());
> return addr;
> }
>
> $ 
> ~/Work/gcc-linaro-aarch64-linux-gnu-4.9-2014.09_linux/bin/aarch64-linux-gnu-gcc
>  -S -x c -O2 x.c
> x.c: In function ‘foo’:
> x.c:5:2: error: impossible constraint in ‘asm’
>   asm("adrp %0, %1" : "=r" (addr) : "S" ());
>   ^
>
> Boo. Following revisions of the compiler work just fine, though.
>
> We can fallback to the "i" constraint for GCC version prior to 5.0,
> which *seems* to do the right thing. Hopefully we will be able to
> remove this at some point, but in the meantime this gets us going.
>
> Signed-off-by: Marc Zyngier 

Acked-by: Ard Biesheuvel 

> ---
> * From v1: Dropped the detection hack and rely on GCC_VERSION
>
>  arch/arm64/include/asm/kvm_asm.h | 8 +++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h 
> b/arch/arm64/include/asm/kvm_asm.h
> index 7ccf770c53d9..8a33d83ea843 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -199,6 +199,12 @@ extern void __vgic_v3_init_lrs(void);
>
>  extern u32 __kvm_get_mdcr_el2(void);
>
> +#if defined(GCC_VERSION) && GCC_VERSION < 5
> +#define SYM_CONSTRAINT "i"
> +#else
> +#define SYM_CONSTRAINT "S"
> +#endif
> +
>  /*
>   * Obtain the PC-relative address of a kernel symbol
>   * s: symbol
> @@ -215,7 +221,7 @@ extern u32 __kvm_get_mdcr_el2(void);
> typeof(s) *addr;\
> asm("adrp   %0, %1\n"   \
> "add%0, %0, :lo12:%1\n" \
> -   : "=r" (addr) : "S" ());  \
> +   : "=r" (addr) : SYM_CONSTRAINT ());   \
> addr;   \
> })
>
> --
> 2.29.2
>
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[PATCH] arm64: Work around broken GCC 4.9 handling of "S" constraint

[Marc Zyngier]

GCC 4.9 seems to have a problem with the "S" asm constraint
when the symbol lives in the same compilation unit, and pretends
the constraint is impossible:

$ cat x.c
void *foo(void)
{
static int x;
int *addr;
asm("adrp %0, %1" : "=r" (addr) : "S" ());
return addr;
}

$ 
~/Work/gcc-linaro-aarch64-linux-gnu-4.9-2014.09_linux/bin/aarch64-linux-gnu-gcc 
-S -x c -O2 x.c
x.c: In function ‘foo’:
x.c:5:2: error: impossible constraint in ‘asm’
  asm("adrp %0, %1" : "=r" (addr) : "S" ());
  ^

Boo. Following revisions of the compiler work just fine, though.

We can fallback to the "i" constraint for GCC version prior to 5.0,
which *seems* to do the right thing. Hopefully we will be able to
remove this at some point, but in the meantime this gets us going.

Signed-off-by: Marc Zyngier 
---
* From v1: Dropped the detection hack and rely on GCC_VERSION

 arch/arm64/include/asm/kvm_asm.h | 8 +++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 7ccf770c53d9..8a33d83ea843 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -199,6 +199,12 @@ extern void __vgic_v3_init_lrs(void);
 
 extern u32 __kvm_get_mdcr_el2(void);
 
+#if defined(GCC_VERSION) && GCC_VERSION < 5
+#define SYM_CONSTRAINT "i"
+#else
+#define SYM_CONSTRAINT "S"
+#endif
+
 /*
  * Obtain the PC-relative address of a kernel symbol
  * s: symbol
@@ -215,7 +221,7 @@ extern u32 __kvm_get_mdcr_el2(void);
typeof(s) *addr;\
asm("adrp   %0, %1\n"   \
"add%0, %0, :lo12:%1\n" \
-   : "=r" (addr) : "S" ());  \
+   : "=r" (addr) : SYM_CONSTRAINT ());   \
addr;   \
})
 
-- 
2.29.2

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