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
