linux-kernel - Re: [PATCH v3 1/6] KVM: nVMX: Use hardware MSR bitmap

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Date:	Wed, 28 Jan 2015 19:43:02 +0800
From:	Wincy Van <fanwenyi0529@...il.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Jan Kiszka <jan.kiszka@....de>
Subject: Re: [PATCH v3 1/6] KVM: nVMX: Use hardware MSR bitmap

On Wed, Jan 28, 2015 at 7:25 PM, Zhang, Yang Z <yang.z.zhang@...el.com> wrote:
> Wincy Van wrote on 2015-01-28:
>> On Wed, Jan 28, 2015 at 4:05 PM, Zhang, Yang Z <yang.z.zhang@...el.com>
>> wrote:
>>>> @@ -8344,7 +8394,68 @@ static int
>>>> nested_vmx_check_msr_bitmap_controls(struct kvm_vcpu *vcpu,  static
>>>> inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>>>>                                                struct vmcs12
>>>> *vmcs12)  { -       return false; +       struct page *page; +
>>>> unsigned long *msr_bitmap; + +       if
>>>> (!nested_cpu_has_virt_x2apic_mode(vmcs12)) +               return
>>>> false; + +       page = nested_get_page(vcpu, vmcs12->msr_bitmap); +
>>>>     if (!page) { +               WARN_ON(1); +               return
>>>> false; +       } +       msr_bitmap = (unsigned long *)kmap(page); +
>>>>     if (!msr_bitmap) { +
>>>> nested_release_page_clean(page); +               WARN_ON(1); +
>>>>       return false; +       } + +       memset(vmx_msr_bitmap_nested,
>>>> 0xff, PAGE_SIZE); + +       if
>>>> (nested_cpu_has_virt_x2apic_mode(vmcs12)) +               /* TPR is
>>>> allowed */ +
>>>> nested_vmx_disable_intercept_for_msr(msr_bitmap, +
>>>>           vmx_msr_bitmap_nested, +
>>>> APIC_BASE_MSR + (APIC_TASKPRI >> 4), +
>>>> MSR_TYPE_R | MSR_TYPE_W);
>>>
>>> I didn't understand what this function does? Per my understanding,
>>> you only
>> need to set the (vmx_msr_bitmap_nested = vmcs01->msr_bitmap |
>> vmcs12->msr_bitmap) and inject the nested vmexit to L1 if the bit in
>> vmcs12->msr_bitmap is setting. Am I missing some patches?
>>
>> In the beginning, I want to do "vmcs01->msr_bitmap |
>> vmcs12->msr_bitmap", but I remember that there isn't a instruction to
>> do a bit or operation in two pages effectively, so I do the bit or
>> operation in nested_vmx_disable_intercept_for_msr. If the hardware do
>> not support this, I think it is faster if we deal with the bits on demand.
>> nested_vmx_merge_msr_bitmap is used to merge L0's and L1's bitmaps,
>> any features can put their logic here.
>
> You construct the nested_msr_bitmap based on vmcs12->msr_bitmap, what happens if vmcs01->msr_bitmap want to trap this msr?
>

If L0 wants to intercept a msr, we should set
vmx_msr_bitmap_legacy(_x2apic) and vmx_msr_bitmap_longmode(_x2apic),
and that bitmaps should only be loaded in non-nested entry.
Currently we only clear the corresponding bits if L1 enables
virtualize x2apic mode, all the other bits are all set. On nested
entry, we load nested_msr_bitmap, on nested vmexit, we will restore
L0's bitmap.
In the nested entry, L0 only care about L2's msr access, not L1's. I
think that in nested entry, nested_msr_bitmap is "vmcs01->msr_bitmap"
as your mentioned above.

Thanks,
Wincy
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