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:

> On Wed, Mar 4, 2015 at 1:39 AM, Bandan Das <bsd@redhat.com> wrote:
>> 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 ?
>>
>
> 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.


> Thanks,
> Wincy
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