linux-kernel - Re: [PATCH v5 4/7] kvm, mem-hotplug: Reload L1' apic access page on migration in vcpu_enter

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Date:	Fri, 12 Sep 2014 11:32:47 +0800
From:	tangchen <tangchen@...fujitsu.com>
To:	Gleb Natapov <gleb@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>
CC:	<mtosatti@...hat.com>, <nadav.amit@...il.com>, <jan.kiszka@....de>,
	<kvm@...r.kernel.org>, <laijs@...fujitsu.com>,
	<isimatu.yasuaki@...fujitsu.com>, <guz.fnst@...fujitsu.com>,
	<linux-kernel@...r.kernel.org>, <tangchen@...fujitsu.com>
Subject: Re: [PATCH v5 4/7] kvm, mem-hotplug: Reload L1' apic access page
 on migration in vcpu_enter_guest().

Hi Gleb, Paolo,

On 09/11/2014 10:47 PM, Gleb Natapov wrote:
> On Thu, Sep 11, 2014 at 04:37:39PM +0200, Paolo Bonzini wrote:
>> Il 11/09/2014 16:31, Gleb Natapov ha scritto:
>>>>> What if the page being swapped out is L1's APIC access page?  We don't
>>>>> run prepare_vmcs12 in that case because it's an L2->L0->L2 entry, so we
>>>>> need to "do something".
>>> We will do something on L2->L1 exit. We will call kvm_reload_apic_access_page().
>>> That is what patch 5 of this series is doing.
>> Sorry, I meant "the APIC access page prepared by L1" for L2's execution.
>>
>> You wrote:
>>
>>> if (!is_guest_mode() || !(vmcs12->secondary_vm_exec_control & ECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES))
>>>      write(PIC_ACCESS_ADDR)
>>>
>>> In other words if L2 shares L1 apic access page then reload, otherwise do nothing.
>> but in that case you have to redo nested_get_page, so "do nothing"
>> doesn't work.
>>
> Ah, 7/7 is new in this submission. Before that this page was still
> pinned.  Looking at 7/7 now I do not see how it can work since it has no
> code for mmu notifier to detect that it deals with such page and call
> kvm_reload_apic_access_page().

Since L1 and L2 share one apic page, if the page is unmapped, 
mmu_notifier will
be called, and :

  - if vcpu is in L1, a L1->L0 exit is rised. apic page's pa will be 
updated in the next
    L0->L1 entry by making vcpu request.

  - if vcpu is in L2 (is_guest_mode, right?), a L2->L0 exit is rised. 
nested_vmx_vmexit()
    will not be called since it is called in L2->L1 exit. It returns 
from vmx_vcpu_run()
    directly, right ? So we should update apic page in L0->L2 entry. 
This is also done
    by making vcpu request, right ?.

    prepare_vmcs02() is called in L1->L2 entry, and nested_vmx_vmexit() 
is called in
    L2->L1 exit. So we also need to update L1's vmcs in 
nested_vmx_vmexit() in patch 5/7.

IIUC, I think patch 1~6 has done such things.

And yes, the is_guest_mode() check is not needed.

> I said to Tang previously that nested
> kvm has a bunch of pinned page that are hard to deal with and suggested
> to iron out non nested case first :(

Yes, and maybe adding patch 7 is not a good idea for now.

Thanks.
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