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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-04-16 18:41, Benjamin 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.
> 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);

You lost most of your whitespaces - in the webmailer? ;)

Jan

> 
>   if (cr4 & X86_CR4_VMXE) {
>   /*
> 

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