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References: <20201014083300.19077-1-ankur.a.arora@oracle.com> <20201014083300.19077-6-ankur.a.arora@oracle.com>
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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 14 Oct 2020 08:45:37 -0700
Message-ID: <CALCETrVKLv5DPByFcj7E5SBbv4mFt7mGQ9j-HU7G5u_aPGCYsQ@mail.gmail.com>
Subject: Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()
To:     Ankur Arora <ankur.a.arora@oracle.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Michal Hocko <mhocko@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
        linux-arch <linux-arch@vger.kernel.org>
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On Wed, Oct 14, 2020 at 1:33 AM Ankur Arora <ankur.a.arora@oracle.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.

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