Re: [PATCH] vt-x: Preserve host CR4.MCE value while in guest mode.

[Bandan Das <bsd@redhat.com>]

Ben Serebrin <serebrin@google.com> writes:

I suggest changing the subject to
KVM: VMX: Preserve host CR4.MCE value while in guest mode
That keeps it consistent with the $SUBJECT naming convention KVM
follows.

If Paolo is ok with changing it in his tree directly, that's fine too.

Bandan

> 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@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) {
>  		/*