Re: [PATCH] KVM: vmx: Set msr bitmap correctly if vcpu is in guest mode

[Bandan Das <bsd@redhat.com>]

Wincy Van <fanwenyi0529@gmail.com> writes:

> In commit 3af18d9c5fe9 ("KVM: nVMX: Prepare for using hardware MSR bitmap"),
> we are setting MSR_BITMAP in prepare_vmcs02 if we should use hardware. This
> is not enough since the field will be modified by following vmx_set_efer.
>
> Fix this by setting vmx_msr_bitmap_nested in vmx_set_msr_bitmap if vcpu is
> in guest mode.
>
> Signed-off-by: Wincy Van <fanwenyi0529@gmail.com>
> ---
>  arch/x86/kvm/vmx.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index f7b20b4..f6e3457 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2168,7 +2168,10 @@ static void vmx_set_msr_bitmap(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long *msr_bitmap;
>  
> -	if (irqchip_in_kernel(vcpu->kvm) && apic_x2apic_mode(vcpu->arch.apic)) {
> +	if (is_guest_mode(vcpu))
> +		msr_bitmap = vmx_msr_bitmap_nested;
> +	else if (irqchip_in_kernel(vcpu->kvm) &&
> +		apic_x2apic_mode(vcpu->arch.apic)) {

So, we end up writing the MSR_BITMAP field twice - once when we
call nested_vmx_merge_msr_bitmap() and another here. Why don't we just
remove the former since prepare_vmcs02 will call vmx_set_efer anyway ?

Bandan

>  		if (is_long_mode(vcpu))
>  			msr_bitmap = vmx_msr_bitmap_longmode_x2apic;
>  		else