2016-08-09 17:32+0800, Yang Zhang:
> On 2016/8/9 2:16, Radim Krčmář wrote:
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> @@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
>> return 1;
>> }
>>
>> + if (cpu_has_vmx_msr_bitmap()) {
>> + vmx->nested.msr_bitmap =
>> + (unsigned long *)__get_free_page(GFP_KERNEL);
>> + if (!vmx->nested.msr_bitmap)
>> + goto out_msr_bitmap;
>> + }
>> +
>
> We export msr_bitmap to guest even it is not supported by hardware. So we
> need to allocate msr_bitmap for L1 unconditionally.
We do emulate the feature, but the vmx->nested.msr_bitmap is used only
if VMX supports it to avoid some VM exits:
>> @@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu,
>> struct vmcs12 *vmcs12)
>> }
>>
>> 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 &&
>> + 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;
The else branch is taken if !cpu_has_vmx_msr_bitmap() and disables msr
bitmaps. Similar check for vmx_set_msr_bitmap(), so the NULL doesn't
even get written to VMCS.
KVM always uses L1's msr bitmaps when emulating the feature.
