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~
