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

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Date:	Wed, 28 Jan 2015 16:24:28 +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 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.


> Best regards,
> Yang
>
>
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