Re: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n

From: Palmer Dabbelt
Date: Fri Aug 10 2018 - 16:55:26 EST

On Fri, 10 Aug 2018 11:47:15 PDT (-0700), linux@xxxxxxxxxxxx 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
>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:

li a7, __NR_riscv_flush_icache
li a0, 0

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.