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

[Wincy Van]

On Wed, Mar 4, 2015 at 12:04 PM, Bandan Das <bsd@xxxxxxxxxx> wrote:
> Wincy Van <fanwenyi0529@xxxxxxxxx> writes:
>
>> On Wed, Mar 4, 2015 at 1:39 AM, Bandan Das <bsd@xxxxxxxxxx> wrote:
>>> Wincy Van <fanwenyi0529@xxxxxxxxx> 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@xxxxxxxxx>
>>>> ---
>>>>  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 ?
>>>
>>
>> Yes, setting MSR_BITMAP twice is redundant, but we can not rely on
>> vmx_set_efer to set that field, this is not vmx_set_efer 's duty.
> It's not. The change is in vmx_set_msr_bitmap() and vmx_set_efer
> happens to call it. The call to the merge function may very well
> belong to prepare_vmcs02() but the write to the vmcs field could
> belong to vmx_set_msr_bitmap.
>
>> Consider that someone wants to make some changes on loading
>> L2's efer, he may be confused about this. We should reduce the
>> degree of code coupling.
> Fine, just add a comment in prepare_vmcs02 that that's where the field
> is being set. No point in doing the same thing twice.
>

Yes, Agreed. I'll send v2 ASAP.

Thanks,
Wincy
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