Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

[Ankur Arora]

On 2020-10-14 8:45 a.m., Andy Lutomirski wrote:
> On Wed, Oct 14, 2020 at 1:33 AM Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
> > Define clear_page_uncached() as an alternative_call() to clear_page_nt()
> > if the CPU sets X86_FEATURE_NT_GOOD and fallback to clear_page() if it
> > doesn't.
> >
> > Similarly define clear_page_uncached_flush() which provides an SFENCE
> > if the CPU sets X86_FEATURE_NT_GOOD.
>
> As long as you keep "NT" or "MOVNTI" in the names and keep functions
> in arch/x86, I think it's reasonable to expect that callers understand
> that MOVNTI has bizarre memory ordering rules.  But once you give
> something a generic name like "clear_page_uncached" and stick it in
> generic code, I think the semantics should be more obvious.
>
> How about:
>
> clear_page_uncached_unordered() or clear_page_uncached_incoherent()
>
> and
>
> flush_after_clear_page_uncached()
>
> After all, a naive reader might expect "uncached" to imply "caches are
> off and this is coherent with everything".  And the results of getting
> this wrong will be subtle and possibly hard-to-reproduce corruption.
Yeah, these are a lot more obvious. Thanks. Will fix.

Ankur

> --Andy