Messages in this thread | | | Date | Sat, 4 Jul 2020 03:30:09 +0100 | From | Al Viro <> | Subject | Re: objtool clac/stac handling change.. |
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On Fri, Jul 03, 2020 at 06:54:15PM -0700, Linus Torvalds wrote:
> "Software Prefetches May Report A Page Fault > > Description Software prefetch instructions are defined to ignore > page faults. Under highly specific and detailed internal > circumstances, a prefetch instruction may report a page fault if both > of the following conditions are true: > > • The target address of the prefetch would cause a page fault if > the address was accessed by an actual memory load or store instruction > under the current privilege mode; > > • The prefetch instruction is followed in execution-order by an > actual or speculative byte-sized memory access of the same > modify-intent to the same address. PREFETCH and PREFETCHNTA/0/1/2 have > the same modify-intent as a memory load access. > > PREFETCHW has the same modify-intent as a memory store access. The > page fault exception error code bits for the faulting prefetch will be > identical to that for a bytesized memory access of the same-modify > intent to the same address. Note that some misaligned accesses can be > broken up by the processor into multiple accesses where at least one > of the accesses is a byte-sized access. If the target address of the > subsequent memory access of the same modify-intent is aligned and not > byte-sized, this errata does not occur and no workaround is needed. > > Potential Effect on System An unexpected page fault may occur > infrequently on a prefetch instruction."
Lovely... So basically this is the rare place where we might use those insns on userland addresses?
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