linux-kernel - [PATCH 1/2] x86: kvmclock: zero initialize pvclock shared memory area

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Date:	Fri, 21 Jun 2013 11:01:20 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	mtosatti@...hat.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, kvm@...r.kernel.org, x86@...nel.org,
	pbonzini@...hat.com, eabatalov89@...il.com
Subject: [PATCH 1/2] x86: kvmclock: zero initialize pvclock shared memory area

Since commit:
  7069ed6 x86: kvmclock: allocate pvclock shared memory area

kernel might hung in pvclock_clocksource_read() due to
uninitialized memory might contain odd version value in
following cycle:

        do {
                version = __pvclock_read_cycles(src, &ret, &flags);
        } while ((src->version & 1) || version != src->version);

if secondary kvmclock is accessed before it's registered with kvm.

Additionally before regression was introduced users of pre-cpu
hv_clock were relying on variable being zero initialized, and
7069ed6 breaks this assumption for usage of:
  hv_clock.migrate_count
which could be populated with random garbage now.

Clearing garbage in pvclock shared memory area right after it's
allocated helps to avoid these issues.

Signed-off-by: Igor Mammedov <imammedo@...hat.com>
---
 arch/x86/kernel/kvmclock.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index d2c3812..3dd37eb 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -242,6 +242,7 @@ void __init kvmclock_init(void)
 	if (!mem)
 		return;
 	hv_clock = __va(mem);
+	memset(hv_clock, 0, size);
 
 	if (kvm_register_clock("boot clock")) {
 		hv_clock = NULL;
-- 
1.7.1

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