The argument passed to cmpxchg is not guaranteed to be sign
extended, but lr.w sign extends on RV64I. This makes cmpxchg
fail on clang built kernels when __old is negative.
To fix this, we just cast __old to long which sign extends on
RV64I. With this fix, clang built RISC-V kernels now boot.
Link: https://github.com/ClangBuiltLinux/linux/issues/867
Cc: [email protected]
Signed-off-by: Nathan Huckleberry <[email protected]>
---
arch/riscv/include/asm/cmpxchg.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index d969bab4a26b..262e5bbb2776 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -179,7 +179,7 @@
" bnez %1, 0b\n" \
"1:\n" \
: "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
- : "rJ" (__old), "rJ" (__new) \
+ : "rJ" ((long)__old), "rJ" (__new) \
: "memory"); \
break; \
case 8: \
@@ -224,7 +224,7 @@
RISCV_ACQUIRE_BARRIER \
"1:\n" \
: "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
- : "rJ" (__old), "rJ" (__new) \
+ : "rJ" ((long)__old), "rJ" (__new) \
: "memory"); \
break; \
case 8: \
@@ -270,7 +270,7 @@
" bnez %1, 0b\n" \
"1:\n" \
: "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
- : "rJ" (__old), "rJ" (__new) \
+ : "rJ" ((long)__old), "rJ" (__new) \
: "memory"); \
break; \
case 8: \
@@ -316,7 +316,7 @@
" fence rw, rw\n" \
"1:\n" \
: "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
- : "rJ" (__old), "rJ" (__new) \
+ : "rJ" ((long)__old), "rJ" (__new) \
: "memory"); \
break; \
case 8: \
--
2.27.0.278.ge193c7cf3a9-goog
On Thu, Jun 11, 2020 at 11:34 AM 'Nathan Huckleberry' via Clang Built
Linux <[email protected]> wrote:
>
> The argument passed to cmpxchg is not guaranteed to be sign
> extended, but lr.w sign extends on RV64I.
I had a hard time finding documentation on this sign extension. Is
lr.w just the atomic version of lw?
https://content.riscv.org/wp-content/uploads/2019/06/riscv-spec.pdf
pdf page 54, printed page 38 says:
The LW instruction loads a 32-bit value from memory and sign-extends
this to 64 bits before storing it in register rd for RV64I.
> This makes cmpxchg
> fail on clang built kernels when __old is negative.
>
> To fix this, we just cast __old to long which sign extends on
> RV64I. With this fix, clang built RISC-V kernels now boot.
Oh, indeed, nice! Thanks for digging into this issue, and sending the patch.
Tested-by: Nick Desaulniers <[email protected]> # QEMU boot, clang build
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/867
> Cc: [email protected]
> Signed-off-by: Nathan Huckleberry <[email protected]>
> ---
> arch/riscv/include/asm/cmpxchg.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index d969bab4a26b..262e5bbb2776 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -179,7 +179,7 @@
> " bnez %1, 0b\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -224,7 +224,7 @@
> RISCV_ACQUIRE_BARRIER \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -270,7 +270,7 @@
> " bnez %1, 0b\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -316,7 +316,7 @@
> " fence rw, rw\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> --
--
Thanks,
~Nick Desaulniers
On Thu, 11 Jun 2020 11:32:35 PDT (-0700), [email protected] wrote:
> The argument passed to cmpxchg is not guaranteed to be sign
> extended, but lr.w sign extends on RV64I. This makes cmpxchg
> fail on clang built kernels when __old is negative.
>
> To fix this, we just cast __old to long which sign extends on
> RV64I. With this fix, clang built RISC-V kernels now boot.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/867
> Cc: [email protected]
> Signed-off-by: Nathan Huckleberry <[email protected]>
> ---
> arch/riscv/include/asm/cmpxchg.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index d969bab4a26b..262e5bbb2776 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -179,7 +179,7 @@
> " bnez %1, 0b\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -224,7 +224,7 @@
> RISCV_ACQUIRE_BARRIER \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -270,7 +270,7 @@
> " bnez %1, 0b\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
> @@ -316,7 +316,7 @@
> " fence rw, rw\n" \
> "1:\n" \
> : "=&r" (__ret), "=&r" (__rc), "+A" (*__ptr) \
> - : "rJ" (__old), "rJ" (__new) \
> + : "rJ" ((long)__old), "rJ" (__new) \
> : "memory"); \
> break; \
> case 8: \
So we talked about this earlier, but just so everyone's one the same page: I
think this should be a compiler bug, but the spec doesn't define any of this
stuff well enough that it actually is. I'm sort of inclined to make it a
compiler bug, but I'm not sure if that's still possible and it requires a lot
more work. I'm writing up a bigger email, but it's been floating around for a
few days and I don't want to delay this on sorting out what our inline assembly
actually does.
I've put this on fixes.
Thanks!