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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 30 Apr 2006, at 02:37, Leendert van Doorn wrote:

> > How does this work now? Do we really have two copies of each device
> > model? I doubt that's implemented safely.
> Right now the realmode code runs inside the VMX partition where it is
> partially emulated by vmxassist. So all accesses to the emulated 
> devices
> go through the hypervisor first before they (potentially) end up in
> qemu-dm. When a transition is made to 32/64-bit code all the 
> initialized
> device state is still there.
Ah yes, I forgot that the mmio decoder stuff in Xen handles real mode. 
So that means that currently each device model is either implemented in 
Xen or in qemu-dm, but not both (now that the heinous split PIT device 
model is gone). That's a nice state of affairs.
> The problem of keeping the the hypervisor state and the qemu-dm state 
> in
> sync is introduced when we alternate between emulation and real
> execution. This becomes more interesting when we consider MP guests
> where one CPU is running inside the emulator and another on the real
> hardware.
It'd obviously be better avoided altogether, unless we have to perform 
horrible contortions to do so, or if doing so would hurt performance of 
operations that we care about.
Don't get me wrong by the way: I do think that leveraging qemu's full 
emulator, at least to get us out of the stickiest situations, is a very 
good idea. I'm only concerned about some of the finer details.
 -- Keir


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