Re: [PATCH v4 4/6] kvm, mem-hotplug: Reload L1' apic access page on migration in vcpu_enter_guest().

[tangchen <tangchen@cn.fujitsu.com>]

Hi Gleb,

On 09/03/2014 11:04 PM, Gleb Natapov wrote:
> On Wed, Sep 03, 2014 at 09:42:30AM +0800, tangchen wrote:
>> Hi Gleb,
>>
>> On 09/03/2014 12:00 AM, Gleb Natapov wrote:
>>> ......
>>> +static void vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
>>> +{
>>> +	/*
>>> +	 * apic access page could be migrated. When the page is being migrated,
>>> +	 * GUP will wait till the migrate entry is replaced with the new pte
>>> +	 * entry pointing to the new page.
>>> +	 */
>>> +	vcpu->kvm->arch.apic_access_page = gfn_to_page(vcpu->kvm,
>>> +				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
>>> +	kvm_x86_ops->set_apic_access_page_addr(vcpu->kvm,
>>> +				page_to_phys(vcpu->kvm->arch.apic_access_page));
>>> I am a little bit worried that here all vcpus write to vcpu->kvm->arch.apic_access_page
>>> without any locking. It is probably benign since pointer write is atomic on x86. Paolo?
>>>
>>> Do we even need apic_access_page? Why not call
>>>   gfn_to_page(APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT)
>>>   put_page()
>>> on rare occasions we need to know its address?
>> Isn't it a necessary item defined in hardware spec ?
>>
> vcpu->kvm->arch.apic_access_page? No. This is internal kvm data structure.
>
>> I didn't read intel spec deeply, but according to the code, the page's
>> address is
>> written into vmcs. And it made me think that we cannnot remove it.
>>
> We cannot remove writing of apic page address into vmcs, but this is not done by
> assigning to vcpu->kvm->arch.apic_access_page, but by vmwrite in set_apic_access_page_addr().

OK, I'll try to remove kvm->arch.apic_access_page and send a patch for 
it soon.

BTW, if you don't have objection to the first two patches, would you 
please help to
commit them first ?

Thanks.

>
> --
> 			Gleb.
> .
>