[PATCH 3.2 050/164] KVM: VMX: Preserve host CR4.MCE value while in guest mode.

[Ben Hutchings]

3.2.70-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Serebrin <serebrin@xxxxxxxxxx>

commit 085e68eeafbf76e21848ad5bafaecec88a11dd64 upstream.

The host's decision to enable machine check exceptions should remain
in force during non-root mode.  KVM was writing 0 to cr4 on VCPU reset
and passed a slightly-modified 0 to the vmcs.guest_cr4 value.

Tested: Built.
On earlier version, tested by injecting machine check
while a guest is spinning.

Before the change, if guest CR4.MCE==0, then the machine check is
escalated to Catastrophic Error (CATERR) and the machine dies.
If guest CR4.MCE==1, then the machine check causes VMEXIT and is
handled normally by host Linux. After the change, injecting a machine
check causes normal Linux machine check handling.

Signed-off-by: Ben Serebrin <serebrin@xxxxxxxxxx>
Reviewed-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.2: use read_cr4() instead of cr4_read_shadow()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/vmx.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3032,8 +3032,16 @@ static void vmx_set_cr3(struct kvm_vcpu
 
 static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
-	unsigned long hw_cr4 = cr4 | (to_vmx(vcpu)->rmode.vm86_active ?
-		    KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);
+	/*
+	 * Pass through host's Machine Check Enable value to hw_cr4, which
+	 * is in force while we are in guest mode.  Do not let guests control
+	 * this bit, even if host CR4.MCE == 0.
+	 */
+	unsigned long hw_cr4 =
+		(read_cr4() & X86_CR4_MCE) |
+		(cr4 & ~X86_CR4_MCE) |
+		(to_vmx(vcpu)->rmode.vm86_active ?
+		 KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);
 
 	if (cr4 & X86_CR4_VMXE) {
 		/*

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