| Message ID | 20201217111135.1536658-1-maz@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: Work around broken GCC 4.9 handling of "S" constraint | expand |
On Thu, 17 Dec 2020 at 12:11, Marc Zyngier <maz@kernel.org> 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" (&x)); > 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" (&x)); > ^ > > 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 <maz@kernel.org> Acked-by: Ard Biesheuvel <ardb@kernel.org> > --- > * 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 < 50000 > +#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" (&s)); \ > + : "=r" (addr) : SYM_CONSTRAINT (&s)); \ > addr; \ > }) > > -- > 2.29.2 >
On Thu, 17 Dec 2020 11:11:35 +0000, 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" (&x)); > 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
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 < 50000 +#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" (&s)); \ + : "=r" (addr) : SYM_CONSTRAINT (&s)); \ addr; \ })
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" (&x)); 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" (&x)); ^ 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 <maz@kernel.org> --- * 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(-)