On Thu, 06 Jul 2017 19:14:25 PDT (-0700), boqun.feng@xxxxxxxxx wrote:
> On Thu, Jul 06, 2017 at 06:04:13PM -0700, Palmer Dabbelt wrote:
> [...]
>> >> +#define __smp_load_acquire(p) \
>> >> +do { \
>> >> + union { typeof(*p) __val; char __c[1]; } __u = \
>> >> + { .__val = (__force typeof(*p)) (v) }; \
>> >> + compiletime_assert_atomic_type(*p); \
>> >> + switch (sizeof(*p)) { \
>> >> + case 1: \
>> >> + case 2: \
>> >> + __u.__val = READ_ONCE(*p); \
>> >> + smb_mb(); \
>> >> + break; \
>> >> + case 4: \
>> >> + __asm__ __volatile__ ( \
>> >> + "amoor.w.aq %1, zero, %0" \
>> >> + : "+A" (*p) \
>> >> + : "=r" (__u.__val) \
>> >> + : "memory"); \
>> >> + break; \
>> >> + case 8: \
>> >> + __asm__ __volatile__ ( \
>> >> + "amoor.d.aq %1, zero, %0" \
>> >> + : "+A" (*p) \
>> >> + : "=r" (__u.__val) \
>> >> + : "memory"); \
>> >> + break; \
>> >> + } \
>> >> + __u.__val; \
>> >> +} while (0)
>> >
>> > 'creative' use of amoswap and amoor :-)
>> >
>> > You should really look at a normal load with ordering instruction
>> > though, that amoor.aq is a rmw and will promote the cacheline to
>> > exclusive (and dirty it).
>>
>> The thought here was that implementations could elide the MW by pattern
>> matching the "zero" (x0, the architectural zero register) forms of AMOs where
>> it's interesting. I talked to one of our microarchitecture guys, and while he
>> agrees that's easy he points out that eliding half the AMO may wreak havoc on
>> the consistency model. Since we're not sure what the memory model is actually
>> going to look like, we thought it'd be best to just write the simplest code
>> here
>>
>> /*
>> * TODO_RISCV_MEMORY_MODEL: While we could emit AMOs for the W and D sized
>> * accesses here, it's questionable if that actually helps or not: the lack of
>> * offsets in the AMOs means they're usually preceded by an addi, so they
>> * probably won't save code space. For now we'll just emit the fence.
>> */
>> #define __smp_store_release(p, v) \
>> ({ \
>> compiletime_assert_atomic_type(*p); \
>> smp_mb(); \
>> WRITE_ONCE(*p, v); \
>> })
>>
>> #define __smp_load_acquire(p) \
>> ({ \
>> union{typeof(*p) __p; long __l;} __u; \
>
> AFAICT, there seems to be an endian issue if you do this. No?
>
> Let us assume typeof(*p) is char and *p == 1, and on a big endian 32bit
> platform:
>
>> compiletime_assert_atomic_type(*p); \
>> __u.__l = READ_ONCE(*p); \
>
> READ_ONCE(*p) is 1 so
> __u.__l is 0x00 00 00 01 now
>
>> smp_mb(); \
>> __u.__p; \
>
> __u.__p is then 0x00.
>
> Am I missing something here?
We're little endian (though I might have still screwed it up). I didn't really
bother looking because...
> Even so why not use the simple definition as in include/asm-generic/barrier.h?
...that's much better -- I forgot there were generic versions, as we used to
have a much more complicated one.
https://github.com/riscv/riscv-linux/commit/910d2bf4c3c349b670a1d839462e32e122ac70a5
Thanks!
