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

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Date:	Wed, 28 Jan 2015 11:09:45 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Wincy Van <fanwenyi0529@...il.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

Wincy Van wrote on 2015-01-28:
> On Wed, Jan 28, 2015 at 4:00 PM, Zhang, Yang Z <yang.z.zhang@...el.com>
> wrote:
>>> @@ -5812,13 +5813,18 @@ static __init int hardware_setup(void)
>>>                                 (unsigned long
>>> *)__get_free_page(GFP_KERNEL);
>>>         if (!vmx_msr_bitmap_longmode_x2apic)
>>>                 goto out4;
>>> +
>>> +       vmx_msr_bitmap_nested = (unsigned long
>>> *)__get_free_page(GFP_KERNEL);
>>> +       if (!vmx_msr_bitmap_nested)
>>> +               goto out5;
>>> +
>> 
>> Since the nested virtualization is off by default. It's better to
>> allocate the page only when nested is true. Maybe adding the following
>> check is better:
>> 
>> if (nested) {
>>         vmx_msr_bitmap_nested = (unsigned long
>>         *)__get_free_page(GFP_KERNEL); if (!vmx_msr_bitmap_nested)
>>                 goto out5;
>> }
> 
> Agreed. Will do.
> 
>> 
>> ...snip...
>> 
>>> +
>>> +/*
>>> + * Merge L0's and L1's MSR bitmap, return false to indicate that
>>> + * we do not use the hardware.
>>> + */
>>> +static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>>> +                                              struct vmcs12
>>> *vmcs12) {
>>> +       return false;
>>> +}
>>> +
>> 
>> The following patches have nothing to do with the MSR control. Why
>> leave the function empty here?
>> 
> 
> No. In patch "Enable nested virtualize x2apic mode", we will return
> false if L1 disabled virt_x2apic_mode, then the hardware MSR-bitmap control is disabled.
> This is faster than rebuilding the vmx_msr_bitmap_nested.
> This function returns false here to indicate that we do not use the hardware.
> Since It is not only virtualize x2apic mode using this, other features
> may use this either. I think it isn't suitable to introduce this function in other patches.

Yes, rebuilding is more costly. But your current implementation cannot leverage the APICv feature correctly. I replied in another thread.

> 
> 
>> Best regards,
>> Yang
>> 
>>


Best regards,
Yang