Re: [Qemu-devel] [PATCH 4/4] membarrier: add --enable-membarrier

[Paolo Bonzini]

On 22/03/2018 02:29, Emilio G. Cota wrote:
> It is indeed hard to see a large impact on performance given the
> large size of our critical sections. But hey, rcu_read_unlock
> goes down from 0.24% to 0.08% of execution time when booting
> aarch64 linux!

I expect something a little better than 0.15% from virtio, especially
once I re-enable MemoryRegionCache in 2.13.

>> +    int main(void) {
>> +        syscall(__NR_membarrier, MEMBARRIER_CMD_QUERY, 0);
>> +        syscall(__NR_membarrier, MEMBARRIER_CMD_SHARED, 0);
>> +    }
> 
> I think we should also check here that MEMBARRIER_CMD_SHARED is
> actually supported;

Checking run-time constraints at compile-time is a bit pointless.  My
idea was to add a third mode where the choice is done at run-time, but
that wouldn't have made 2.12.

Paolo