linux-kernel - [PATCH] perf/x86: fix microcode revision check for SNB-PEBS

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Message-ID: <20120824133434.GA8014@quad>
Date:	Fri, 24 Aug 2012 15:34:34 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...e.hu, andi@...stfloor.org,
	borislav.petkov@....com
Subject: [PATCH] perf/x86: fix microcode revision check for SNB-PEBS

The following patch, relative to 3.6.0-rc3, makes
the microcode update code path actually invoke the
perf_check_microcode() function and thus potentially
renabling SNB PEBS.

By default, CONFIG_MICROCODE_OLD_INTERFACE is
forced to Y in arch/x86/Kconfig. There is no
way to disable this. That means that the code
path used in arch/x86/kernel/microcode_core.c
did not include the call to perf_check_microcode().

Thus, even though the microcode was updated to a
version that fixes the SNB PEBS problem, perf_event
would still return EOPNOTSUPP when enabling precise
sampling.

This patch simply adds a call to perf_check_microcode()
in the call path used when OLD_INTERFACE=y.

Signed-off-by: Stephane Eranian <eranian@...gle.com>
---

--- a/arch/x86/kernel/microcode_core.c
+++ b/arch/x86/kernel/microcode_core.c
@@ -225,6 +225,9 @@ static ssize_t microcode_write(struct file *file, const char __user *buf,
 	if (do_microcode_update(buf, len) == 0)
 		ret = (ssize_t)len;

+	if (ret > 0)
+		perf_check_microcode();
+
 	mutex_unlock(&microcode_mutex);
 	put_online_cpus();

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