linux-kernel - Re: [PATCH v2] KVM: vmx: Set msr bitmap correctly if vcpu is in guest mode

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Date:	Thu, 5 Mar 2015 10:53:34 +0800
From:	Wincy Van <fanwenyi0529@...il.com>
To:	Bandan Das <bsd@...hat.com>
Cc:	mtosatti@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Kiszka <jan.kiszka@....de>,
	Yong Wang <yong.y.wang@...ux.intel.com>
Subject: Re: [PATCH v2] KVM: vmx: Set msr bitmap correctly if vcpu is in guest mode

On Wed, Mar 4, 2015 at 11:58 PM, Bandan Das <bsd@...hat.com> wrote:
> Hi Wincy,
>
> Wincy Van <fanwenyi0529@...il.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@...il.com>
>> ---
>>  arch/x86/kvm/vmx.c |   11 +++++++----
>>  1 files changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index f7b20b4..10a481b 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;
> Do you think this should be (is_guest_mode(vcpu) &
>                                   (exec_control &  CPU_BASED_USE_MSR_BITMAPS)) ?
>

We don't need to do that, because if we don't use the hardware, we will disable
hardware msr bitmap:

        if (cpu_has_vmx_msr_bitmap() &&
            exec_control & CPU_BASED_USE_MSR_BITMAPS) {
                nested_vmx_merge_msr_bitmap(vcpu, vmcs12);
                /* MSR_BITMAP will be set by following vmx_set_efer. */
        } else
                exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;

Then, the hardware will ignore MSR_BITMAP.
Setting MSR_BITMAP without enabling it may be unnecessary, but it is no
harm, and we can reduce the code complexity of vmx_set_msr_bitmap.


Thanks,
Wincy
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