linux-kernel - Re: [PATCH 5/8] x86/clear_page: add clear_page

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Message-ID: <20201014195823.GC18196@zn.tnic>
Date:   Wed, 14 Oct 2020 21:58:23 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

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@...cle.com> 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?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette