linux-kernel - Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

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Message-ID: <alpine.DEB.2.21.1907151020320.1669@nanos.tec.linutronix.de>
Date:   Mon, 15 Jul 2019 10:24:21 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Uros Bizjak <ubizjak@...il.com>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop
 targets

Uros,

On Thu, 11 Jul 2019, Uros Bizjak wrote:
> Recent patch [1] disabled a self-snoop feature on a list of processor
> models with a known errata, so we are confident that the feature
> should work on remaining models also for other purposes than to speed
> up MTRR programming.
> 
> I would like to resurrect an old patch [2] that avoids calling clflush
> and wbinvd
> to invalidate caches when CPU supports selfsnoop.

Please do not attach patches, send them inline and please add a proper
changelog. Just saying 'Disable CPA cache flush for selfsnoop targets' in
the subject line then nada gives absolutely zero information.
 
> The patch was ported to latest Fedora kernel (5.1.16) and tested with
> CONFIG_CPA_DEBUG on INTEL_FAM6_IVYBRIDGE_X. The relevant ports of
> dmesg show:
> 
> ...
> < hundreds of CPA protect messages, resulting from set_memory_rw CPA
> undo test in mm/init_64.c >
> CPA  protect  Rodata RO: 0xffffffffbd1fe000 - 0xffffffffbd1fefff PFN
> 1461fe req 8000000000000063 prevent 0000000000000002
> CPA  protect  Rodata RO: 0xffff889c461fe000 - 0xffff889c461fefff PFN
> 1461fe req 8000000000000063 prevent 0000000000000002
> Testing CPA: again
> Freeing unused kernel image memory: 2016K
> Freeing unused kernel image memory: 4K
> x86/mm: Checked W+X mappings: passed, no W+X pages found.
> rodata_test: all tests were successful
> x86/mm: Checking user space page tables
> x86/mm: Checked W+X mappings: passed, no W+X pages found.
> 
> and from CPA selftest:
> 
> CPA self-test:
>  4k 36352 large 4021 gb 0 x 81[ffff889b00098000-ffff889bdf7ff000] miss 133120
>  4k 180224 large 3740 gb 0 x 81[ffff889b00098000-ffff889bdf7ff000] miss 133120
>  4k 180224 large 3740 gb 0 x 81[ffff889b00098000-ffff889bdf7ff000] miss 133120
> ok.

These outputs are pretty useless simply because the selftest only verifies
the inner workings of CPA itself, but has nothing to do with the
correctness vs. cache flushing.

Thanks,

	tglx