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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue, 7 Aug 2012 08:36:10 +0000 (UTC)
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Subject: svn commit: r239125 - head/sys/amd64/amd64
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Author: kib
Date: Tue Aug  7 08:36:10 2012
New Revision: 239125
URL: http://svn.freebsd.org/changeset/base/239125

Log:
  Do not apply errata 721 workaround when under hypervisor, since
  typical hypervisor does not implement access to the required MSR,
  causing #GP on boot.
  
  Reported and tested by:	olgeni
  PR:	amd64/170388
  MFC after:	3 days

Modified:
  head/sys/amd64/amd64/initcpu.c

Modified: head/sys/amd64/amd64/initcpu.c
==============================================================================
--- head/sys/amd64/amd64/initcpu.c	Tue Aug  7 07:52:25 2012	(r239124)
+++ head/sys/amd64/amd64/initcpu.c	Tue Aug  7 08:36:10 2012	(r239125)
@@ -91,11 +91,17 @@ init_amd(void)
 	 *
 	 * http://support.amd.com/us/Processor_TechDocs/41322_10h_Rev_Gd.pdf
 	 * http://support.amd.com/us/Processor_TechDocs/44739_12h_Rev_Gd.pdf
+	 *
+	 * Hypervisors do not provide access to the errata MSR,
+	 * causing #GP exception on attempt to apply the errata.  The
+	 * MSR write shall be done on host and persist globally
+	 * anyway, so do not try to do it when under virtualization.
 	 */
 	switch (CPUID_TO_FAMILY(cpu_id)) {
 	case 0x10:
 	case 0x12:
-		wrmsr(0xc0011029, rdmsr(0xc0011029) | 1);
+		if ((cpu_feature2 & CPUID2_HV) == 0)
+			wrmsr(0xc0011029, rdmsr(0xc0011029) | 1);
 		break;
 	}
 }