linux-kernel - Re: [PATCH v2 5/5] kvm, mem-hotplug: Do not pin apic access page in memory.

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Date:	Tue, 15 Jul 2014 20:54:01 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Gleb Natapov <gleb@...antech.com>
CC:	Jan Kiszka <jan.kiszka@....de>, <mtosatti@...hat.com>,
	<nadav.amit@...il.com>, <kvm@...r.kernel.org>,
	<laijs@...fujitsu.com>, <isimatu.yasuaki@...fujitsu.com>,
	<guz.fnst@...fujitsu.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] kvm, mem-hotplug: Do not pin apic access page
 in memory.

On 07/15/2014 08:40 PM, Gleb Natapov wrote:
> On Tue, Jul 15, 2014 at 08:28:22PM +0800, Tang Chen wrote:
>> On 07/15/2014 08:09 PM, Gleb Natapov wrote:
>>> On Tue, Jul 15, 2014 at 01:52:40PM +0200, Jan Kiszka wrote:
>> ......
>>>>
>>>> I cannot follow your concerns yet. Specifically, how should
>>>> APIC_ACCESS_ADDR (the VMCS field, right?) change while L2 is running? We
>>>> currently pin/unpin on L1->L2/L2->L1, respectively. Or what do you mean?
>>>>
>>> I am talking about this case:
>>>           if (cpu_has_secondary_exec_ctrls()) {a
>>>           } else {
>>>               exec_control |=
>>>                  SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
>>>              vmcs_write64(APIC_ACCESS_ADDR,
>>>                  page_to_phys(vcpu->kvm->arch.apic_access_page));
>>>           }
>>> We do not pin here.
>>>
>>
>> Hi Gleb,
>>
>>
>> 7905                 if (exec_control&
>> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES) {
>> ......
>> 7912                         if (vmx->nested.apic_access_page) /* shouldn't
>> happen */
>> 7913 nested_release_page(vmx->nested.apic_access_page);
>> 7914                         vmx->nested.apic_access_page =
>> 7915                                 nested_get_page(vcpu,
>> vmcs12->apic_access_addr);
>>
>> I thought you were talking about the problem here. We pin
>> vmcs12->apic_access_addr
>> in memory. And I think we should do the same thing to this page as to L1 vm.
>> Right ?
> Nested kvm pins a lot of pages, it will probably be not easy to handle all of them,
> so for now I am concerned with non nested case only (but nested should continue to
> work obviously, just pin pages like it does now).

True. I will work on it.

And also, when using PCI passthrough, kvm_pin_pages() also pins some 
pages. This is
also in my todo list.

But sorry, a little strange. I didn't find where 
vmcs12->apic_access_addr is allocated
or initialized... Would you please tell me ?

>
>>
>> ......
>> 7922                         if (!vmx->nested.apic_access_page)
>> 7923                                 exec_control&=
>> 7924 ~SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
>> 7925                         else
>> 7926                                 vmcs_write64(APIC_ACCESS_ADDR,
>> 7927 page_to_phys(vmx->nested.apic_access_page));
>> 7928                 } else if
>> (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)) {
>> 7929                         exec_control |=
>> 7930 SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
>> 7931                         vmcs_write64(APIC_ACCESS_ADDR,
>> 7932 page_to_phys(vcpu->kvm->arch.apic_access_page));
>> 7933                 }
>>
>> And yes, we have the problem you said here. We can migrate the page while L2
>> vm is running.
>> So I think we should enforce L2 vm to exit to L1. Right ?
>>
> We can request APIC_ACCESS_ADDR reload during L2->L1 vmexit emulation, so
> if APIC_ACCESS_ADDR changes while L2 is running it will be reloaded for L1 too.
>

apic pages for L2 and L1 are not the same page, right ?

I think, just like we are doing in patch 5/5, we cannot wait for the 
next L2->L1 vmexit.
We should enforce a L2->L1 vmexit in mmu_notifier, just like 
make_all_cpus_request() does.

Am I right ?

Thanks.

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