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

[Ankur Arora]

On 2020-10-14 2:07 p.m., Andy Lutomirski wrote:
> > On Oct 14, 2020, at 12:58 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > On Wed, Oct 14, 2020 at 08:45:37AM -0700, 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.
> >
> > Why does it have to be a separate call? Why isn't it behind the
> > clear_page() alternative machinery so that the proper function is
> > selected at boot? IOW, why does a user of clear_page functionality need
> > to know at all about an "uncached" variant?
>
> I assume it’s for a little optimization of clearing more than one page
> per SFENCE.
>
> In any event, based on the benchmark data upthread, we only want to do
> NT clears when they’re rather large, so this shouldn’t be just an
> alternative. I assume this is because a page or two will fit in cache
> and, for most uses that allocate zeroed pages, we prefer cache-hot
> pages. When clearing 1G, on the other hand, cache-hot is impossible
> and we prefer the improved bandwidth and less cache trashing of NT
> clears.

Also, if we did extend clear_page() to take the page-size as parameter
we still might not have enough information (ex. a 4K or a 2MB page that
clear_page() sees could be part of a GUP of a much larger extent) to
decide whether to go uncached or not.

> Perhaps SFENCE is so fast that this is a silly optimization, though,
> and we don’t lose anything measurable by SFENCEing once per page.
Alas, no. I tried that and dropped about 15% performance on Rome.

Thanks
Ankur