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

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Date:	Wed, 28 Jan 2015 20:06:20 +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 7:52 PM, Zhang, Yang Z <yang.z.zhang@...el.com> wrote:
>>>
>>
>> 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.
>
> Mmm... I think if the bit is setting in vmcs01->msr_bitmap means whenever the VCPU(no matter in nested guest mode or not) accesses this msr should cause vmexit, not only L1. That's why need to construct the vmcs02->msr_bitmap based on (vmcs01->msr_bitmap ORed vmcs12->msr_bitmap).
>

You are right, but this is not fit for all the cases, we should custom
the nested_msr_bitmap.
e.g.  Currently L0 wants to intercept some of the x2apic msrs' reading:
         if (enable_apicv) {
                for (msr = 0x800; msr <= 0x8ff; msr++)
                        vmx_disable_intercept_msr_read_x2apic(msr);

                /* According SDM, in x2apic mode, the whole id reg is used.
                 * But in KVM, it only use the highest eight bits. Need to
                 * intercept it */
                vmx_enable_intercept_msr_read_x2apic(0x802);
                /* TMCCT */
                vmx_enable_intercept_msr_read_x2apic(0x839);
                /* TPR */
                vmx_disable_intercept_msr_write_x2apic(0x808);
                /* EOI */
                vmx_disable_intercept_msr_write_x2apic(0x80b);
                /* SELF-IPI */
                vmx_disable_intercept_msr_write_x2apic(0x83f);
        }
But L1 may not want this. So I think we are better to deal with the msrs
seperately, there is not a common way suit for all the cases. If other
features want to intercept a msr in nested entry, they can put the
custom code in nested_vmx_merge_msr_bitmap.


Thanks,
Wincy
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