On 19/05/2023 10:12, Ryan Roberts wrote: > On 18/05/2023 20:28, Yu Zhao wrote: >> On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: >>> >>> There are many call sites that directly dereference a pte_t pointer. >>> This makes it very difficult to properly encapsulate a page table in the >>> arch code without having to allocate shadow page tables. ptep_deref() >>> aims to solve this by replacing all direct dereferences with a call to >>> this function. >>> >>> The default implementation continues to just dereference the pointer >>> (*ptep), so generated code should be exactly the same. However, it is >>> possible for the architecture to override the default with their own >>> implementation, that can (e.g.) hide certain bits from the core code, or >>> determine young/dirty status by mixing in state from another source. >>> >>> While ptep_get() and ptep_get_lockless() already exist, these are >>> implemented as atomic accesses (e.g. READ_ONCE() in the default case). >>> So rather than using ptep_get() and risking performance regressions, >>> introduce an new variant. >> >> We should reuse ptep_get(): >> 1. I don't think READ_ONCE() can cause measurable regressions in this case. >> 2. It's technically wrong without it. > > Can you clarify what you mean by technically wrong? Are you saying that the > current code that does direct dereferencing is buggy? > > I previously convinced myself that the potential for the compiler generating > multiple loads was safe because the code in question is under the PTL so there > are no concurrent stores. And we shouldn't see any tearing for the same reason. > > That said, if there is concensus that we can just use ptep_get() (== > READ_ONCE()) everywhere, then I agree that would be cleaner. Does anyone object? Hi all, A politie bump: It would be great to hear opinions on this before I go ahead and make the change. Thanks, Ryan
