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 15:40:48 +0300
From:	Gleb Natapov <gleb@...antech.com>
To:	Tang Chen <tangchen@...fujitsu.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 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).

> 
> ......
> 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.

--
			Gleb.
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