linux-kernel - RE: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter

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Message-ID: <65cdd5f19431423dac13fbb13719ba55@intel.com>
Date:   Mon, 30 Aug 2021 15:41:45 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        Borislav Petkov <bp@...en8.de>, X86-ML <x86@...nel.org>
Subject: RE: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

>> No #MC on stores. Just on loads. Note that you can't clear poison
>> state with a series of small writes to the cache line. But a single
>> 64-byte store might do it (architects didn't want to guarantee that
>> it would work when I asked about avx512 stores to clear poison
>> many years ago).
>
> Dave Jiang thinks MOVDIR64B clears poison.
>
> http://archive.lwn.net:8080/linux-kernel/157617505636.42350.1170110675242558018.stgit@...ang5-desk3.ch.intel.com/

MOVDIR64B has some explicit guarantees (does a write-back invalidate if the target is already
in the cache) that a 64-byte avx512 write doesn't.

Of course it would stop working if some future CPU were to have a longer than 64 bytes cache line.

-Tony