Messages in this thread | | | Date | Fri, 10 Aug 2018 13:55:15 -0700 (PDT) | Subject | Re: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n | From | Palmer Dabbelt <> |
| |
On Fri, 10 Aug 2018 11:47:15 PDT (-0700), linux@roeck-us.net wrote: > On Fri, Aug 10, 2018 at 11:27:37AM -0700, Palmer Dabbelt wrote: >> On Fri, 10 Aug 2018 01:38:04 PDT (-0700), Christoph Hellwig wrote: >> >On Thu, Aug 09, 2018 at 03:19:51PM -0700, Palmer Dabbelt wrote: >> >>This would be necessary to make non-SMP builds work, but there is >> >>another error in the implementation of our syscall linkage that actually >> >>just causes sys_riscv_flush_icache to never build. I've build tested >> >>this on allnoconfig and allnoconfig+SMP=y, as well as defconfig like >> >>normal. >> > >> >Would't it make sense to use COND_SYSCALL to stub out the syscall >> >for !SMP builds? >> >> I'm not sure. We can implement the syscall fine in !SMP, it's just that the >> vDSO is expected to always eat these calls because in non-SMP mode you can >> do a global fence.i by just doing a local fence.i (there's only one hart). >> >> The original rationale behind not having the syscall in non-SMP mode was to >> limit the user ABI, but on looking again that seems like it's just a bit of >> extra complexity that doesn't help anything. It's already been demonstrated > > Doesn't this mean that some userspace code will only run if the kernel was > compiled for SMP ? I always thought that was unacceptable.
Well, the officially sanctioned way to obtain this functionality is via a vDSO call. On non-SMP systems it will never make the system call. As a result we thought we'd keep it out of the ABI, but after looking again it seems yucky to do so. Here's the vDSO entry, for reference:
ENTRY(__vdso_flush_icache) .cfi_startproc #ifdef CONFIG_SMP li a7, __NR_riscv_flush_icache ecall #else fence.i li a0, 0 #endif ret .cfi_endproc ENDPROC(__vdso_flush_icache)
Note that glibc has a fallback to make the system call if it can't find the vDSO entry, but then doesn't have a secondary fallback to emit a local fence.i if the system call doesn't exist. It seems easier to fix the kernel to always provide the syscall and just call it a bug.
| |