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:	Thu, 11 Sep 2014 16:59:56 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Tang Chen <tangchen@...fujitsu.com>, 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
Subject: Re: [PATCH v5 4/7] kvm, mem-hotplug: Reload L1' apic access page on
 migration in vcpu_enter_guest().

On Thu, Sep 11, 2014 at 03:05:05PM +0200, Paolo Bonzini wrote:
> Il 11/09/2014 13:30, Gleb Natapov ha scritto:
> >> > +			vmcs_write64(APIC_ACCESS_ADDR, page_to_phys(page));
> >> > +			/*
> >> > +			 * Do not pin apic access page in memory so that memory
> >> > +			 * hotplug process is able to migrate it.
> >> > +			 */
> >> > +			put_page(page);
> >> >  		}
> > This code is in prepare_vmcs02() and is executed during L1->L2 vmentry. What happens
> > when apic access page is migrated while L2 is running? It needs to be update somewhere.
> 
> Before it is migrated, the MMU notifier is called and will force a
> vmexit on all CPUs.  The reload code will call GUP again on the page
> again and swap it in.
> 
This is how it will work without "if (!is_guest_mode(vcpu))". But,
unless I am missing something, with this check it will not work while
vcpu is in L2.

Suppose vmcs01->APIC_ACCESS_ADDR = 0xf000. During L2 entry
vmcs02->APIC_ACCESS_ADDR is set to 0xf000 too (by prepare_vmcs02). Now
0xf000 is migrated to 0x8000, mmu notifier is called, it forces vmexit,
but vcpu is in a guest mode so vmcs02->APIC_ACCESS_ADDR is never updated
to 0x8000 because of "if (!is_guest_mode(vcpu))" check. So what am I
missing here?

--
			Gleb.
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