linux-kernel - [PATCH 1/4] KVM: x86: remove unnecessary memory barriers for shared MSRs

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Message-Id: <1438176538-26191-2-git-send-email-pbonzini@redhat.com>
Date:	Wed, 29 Jul 2015 15:28:55 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	mtosatti@...hat.com, srutherford@...gle.com, rkrcmar@...hat.com
Subject: [PATCH 1/4] KVM: x86: remove unnecessary memory barriers for shared MSRs

There is no smp_rmb matching the smp_wmb.  shared_msr_update is called from
hardware_enable, which in turn is called via on_each_cpu.  on_each_cpu
and must imply a read memory barrier (on x86 the rmb is achieved simply
through asm volatile in native_apic_mem_write).

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/x86.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c675ea3351cf..2d62229aac26 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -222,11 +222,9 @@ static void shared_msr_update(unsigned slot, u32 msr)
 void kvm_define_shared_msr(unsigned slot, u32 msr)
 {
 	BUG_ON(slot >= KVM_NR_SHARED_MSRS);
+	shared_msrs_global.msrs[slot] = msr;
 	if (slot >= shared_msrs_global.nr)
 		shared_msrs_global.nr = slot + 1;
-	shared_msrs_global.msrs[slot] = msr;
-	/* we need ensured the shared_msr_global have been updated */
-	smp_wmb();
 }
 EXPORT_SYMBOL_GPL(kvm_define_shared_msr);
 
-- 
1.8.3.1


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