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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, Fenghua Yu <fenghua.yu@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        =?UTF-8?q?Jan=20H=20=2E=20Sch=C3=B6nherr?= <jschoenh@amazon.de>,
        David Duncan <davdunc@amazon.com>
Subject: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host issue
Date:   Thu, 20 Dec 2018 14:40:46 +0100
Message-Id: <20181220134046.7916-1-vkuznets@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

It was found that AWS x1 instances (Xen-based) lack xen.git commit
1f1d183d4900 (x86/HVM: don't give the wrong impression of WRMSR succeeding)
and because of that the wrmsr_safe() check in cache_alloc_hsw_probe()
doesn't help: the consequent rdmsr() blows up with

 unchecked MSR access error: RDMSR from 0xc90 at rIP:
   0xffffffff88c5bba3 (native_read_msr+0x3/0x30)

The issue should definitely get fixed on AWS side. We can, however, simply
workaround this in Linux and live happily after.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kernel/cpu/intel_rdt.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/intel_rdt.c b/arch/x86/kernel/cpu/intel_rdt.c
index 44272b7107ad..0acee6cd07a8 100644
--- a/arch/x86/kernel/cpu/intel_rdt.c
+++ b/arch/x86/kernel/cpu/intel_rdt.c
@@ -213,7 +213,8 @@ static inline void cache_alloc_hsw_probe(void)
 
 	if (wrmsr_safe(IA32_L3_CBM_BASE, max_cbm, 0))
 		return;
-	rdmsr(IA32_L3_CBM_BASE, l, h);
+	if (rdmsr_safe(IA32_L3_CBM_BASE, &l, &h))
+		return;
 
 	/* If all the bits were set in MSR, return success */
 	if (l != max_cbm)
-- 
2.19.2