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 17:04:15 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Jan Kiszka <jan.kiszka@....de>
Cc:	Tang Chen <tangchen@...fujitsu.com>, 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 03:10:15PM +0200, Jan Kiszka wrote:
> On 2014-07-15 14:40, Gleb Natapov wrote:
> >>
> >> ......
> >> 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.
> 
> How should this host-managed VMCS field possibly change while L2 is running?
> 
That what "Do not pin apic access page in memory" patch is doing. It changes APIC_ACCESS_ADDR
of a current vmcs. It kicks vcpu out of a guest mode of course, but vcpu may still be in 
L2 at this point, so only L2 vmcs will get new APIC_ACCESS_ADDR pointer, L1 vmcs will still have
an old one.
   
--
			Gleb.
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