Re: [PATCH] target/hppa: Optimize ldcw/ldcd instruction translation

[Richard Henderson <richard.henderson@linaro.org>]

On 9/13/23 10:19, Helge Deller wrote:
> On 9/13/23 18:55, Richard Henderson wrote:
>> On 9/13/23 07:47, Helge Deller wrote:
>>> +        haddr = (uint32_t *)((uintptr_t)vaddr);
>>> +        old = *haddr;
>>
>> This is horribly incorrect, both for user-only and system mode.
> 
> Richard, thank you for the review!
> But would you mind explaining why this is incorrect?
> I thought the "vaddr = probe_access()" calculates the host address, so
> shouldn't it be the right address?

The vaddr name is confusing (since it implies virtual address, which the return from 
probe_access is not) as are the casts, which are unnecessary.



> 
>>> +        /* if already zero, do not write 0 again to reduce memory presssure */
>>> +        if (old == 0) {
>>> +            return 0;
>>> +        }
>>> +        old = qatomic_xchg(haddr, (uint32_t) 0);
>>
>> You're also dropping the required host memory barrier.
> 
> ?

The path through the read+test+return, without the qatomic_xchg, has no host memory 
barrier to provide sequential consistency of the entire operation.


r~