# HG changeset patch
# User Bandan Das
# Date 1463518351 14400
# Tue May 17 16:52:31 2016 -0400
# Node ID 48936dfc0d232660db89ac2de7f924195f2b3760
# Parent 1ed81e15773346e5bfb6da253f7476f37ceeea3d
# v2: Move the check up since g_cpuid_ext_feat_info is accessed before
# the call to supports_vmx, also check for smx support
We found this problem when booting a KVM guest through tboot from a
host OS where the VMX register is not exposed to the guest, even when
the guest has cloned the host CPU.
Attempting to read MSR_IA32_FEATURE_CONTROL before checking whether it
exists, on CPUs where it does not exist, sends the BSP into an infinite
loop. #GP is asserted when trying to read the non-existent MSR, which
resets the IP, only to again encounter the attempted read of the
non-existent MSR.
Postponing the read of MSR_IA32_FEATURE_CONTROL until the existence of
VMX/SMX has been ascertained prevents this problem.
Signed-off-by: Tony Camuso <tcam...@redhat.com>
Signed-off-by: Bandan Das <b...@redhat.com>
diff -r 1ed81e157733 -r 48936dfc0d23 tboot/txt/verify.c
--- a/tboot/txt/verify.c Wed Apr 20 16:31:18 2016 -0700
+++ b/tboot/txt/verify.c Tue May 17 16:52:31 2016 -0400
@@ -109,8 +109,13 @@
}
g_cpuid_ext_feat_info = cpuid_ecx(1);
- g_feat_ctrl_msr = rdmsr(MSR_IA32_FEATURE_CONTROL);
- printk(TBOOT_DETA"IA32_FEATURE_CONTROL_MSR: %08lx\n", g_feat_ctrl_msr);
+ /* read feature control msr only if processor supports VMX or SMX
instructions */
+ if ( (g_cpuid_ext_feat_info & CPUID_X86_FEATURE_VMX) ||
+ (g_cpuid_ext_feat_info & CPUID_X86_FEATURE_SMX) ) {
+ g_feat_ctrl_msr = rdmsr(MSR_IA32_FEATURE_CONTROL);
+ printk(TBOOT_DETA"IA32_FEATURE_CONTROL_MSR: %08lx\n",
g_feat_ctrl_msr);
+ }
+
return true;
}
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