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Re: [Xen-devel] [PATCH] Calculate correct instruction length for data-fa

To: leendert@xxxxxxxxxxxxxx
Subject: Re: [Xen-devel] [PATCH] Calculate correct instruction length for data-fault VM exits on VT-x systems
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Sat, 29 Apr 2006 09:00:50 +0100
Cc: "Petersson, Mats" <Mats.Petersson@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Khoa Huynh <khoa@xxxxxxxxxx>
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On 29 Apr 2006, at 02:20, Leendert van Doorn wrote:

The only difficulty is that the hypervisor keeps some of the device
state vpit and *pics and shotcuts operations to them. This state needs
to be exposed to qemu-dm so that it is saved and restored on every
qemu-dm invocation. I need to verify this, but as far as I'm aware, all
the accesses to the devices emulated in the hypervisor are PIO
operations. These are easy to decode with the exit information that is
provided by  VT-x and SVM, so they don't need a a full instruction

The APIC and IO-APIC are accessed via mmio. The former is written fairly frequently with singleton updates (to the TPR and EOI registers) so we'd want to carry on dealing with those directly in Xen I should think. Still you'd have to deal with the case that one of the Xen-emulated devices is accessed while emulating in qemu-dm -- as you say you'd probably have to pull their state vectors out of Xen when starting emulating. We'll need that for save/restore anyway though.

I don't know if this will make sense for emulated I/O but it does sound like a very sane alternative to vmxassist for dealing with real mode.

 -- Keir

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