Re: [Xen-devel] [PATCH] Calculate correct instruction length for data-fault VM exits on VT-x systems

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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
> decoder.
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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