Re: [PATCH] KVM: VMX: Preserve host CR4.MCE value while in guest mode.

[Wanpeng Li <wanpeng.li@linux.intel.com>]

Hi Ben,
On Thu, Apr 16, 2015 at 11:58:05AM -0700, Ben Serebrin wrote:
>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.

Could you point out which section of SDM describes that the machine check 
is escalated to a CATERR if CR4.MCE==0?

Regards,
Wanpeng Li 

>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@google.com>
>---
> arch/x86/kvm/vmx.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
>diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>index f5e8dce..f7b6168 100644
>--- a/arch/x86/kvm/vmx.c
>+++ b/arch/x86/kvm/vmx.c
>@@ -3622,8 +3622,16 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> 
> 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 =
>+		(cr4_read_shadow() & 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) {
> 		/*
>-- 
>2.2.0.rc0.207.ga3a616c
>
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