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

[Andy Lutomirski]

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.

--Andy