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Re: [Xen-devel] [PATCH] VMX virtual-apic: Update the p2m mapping alongsi

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] VMX virtual-apic: Update the p2m mapping alongside the control bit
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 26 May 2010 08:22:43 +0100
Delivery-date: Wed, 26 May 2010 00:23:42 -0700
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Thread-topic: [Xen-devel] [PATCH] VMX virtual-apic: Update the p2m mapping alongside the control bit
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On 25/05/2010 15:42, "Tim Deegan" <Tim.Deegan@xxxxxxxxxx> wrote:

> Otherwise a VM migrated from a system which supports the feature to one
> which doesn't ends up with a data page where its vlapic should be.

I don't see that this problem exists, let alone that this patch would fix it
if it did.

The dummy data page should not be migrated because it has p2m type
mmio_direct, which will fail the p2m_is_ram() check in mod_l1_entry() when
dom0 attempts to map the page to save/migrate it.

Even if that weren't the case, the dummy page will be mapped into the
guest's p2m unless the apic is disabled or relocated from its default
physaddr location. Neither of these is especially likely to be true during

The only thing I can see is this patch avoids a dummy page hanging around in
the p2m when HW APIC virt is disabled. But the dummy page is pretty benign
in this situation, so I don't see that we care.

 -- Keir

> Signed-off-by: Tim Deegan <Tim.Deegan@xxxxxxxxxx>

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