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

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Date:   Thu, 11 Jul 2019 10:12:55 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Andrew Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

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.

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.

[1] https://lkml.org/lkml/2019/6/27/828
[2] https://lkml.org/lkml/2009/4/8/508

Uros.

View attachment "0001-Disable-CPA-cache-flush-for-selfsnoop-targets.patch" of type "text/x-patch" (1003 bytes)