Re: [Xen-devel] HYBRID: PV in HVM container

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 29 Jul 2011, Konrad Rzeszutek Wilk wrote:
> > Besides if we have HVM dom0, we can enable
> > XENFEAT_auto_translated_physmap and EPT and have the same level of
> > performances of a PV on HVM guest. Moreover since we wouldn't be using
> > the mmu pvops anymore we could drop them completely: that would greatly
> 
> Sure. It also means you MUST have an IOMMU in the box.

Why?
We can still remap interrupts into event channels.
Maybe you mean VMX?


> > simplify the Xen maintenance in the Linux kernel as well as gain back
> > some love from the x86 maintainers :)
> > 
> > The way I see it, normal Linux guests would be PV on HVM guests, but we
> > still need to do something about dom0.
> > This work would make dom0 exactly like PV on HVM guests apart from
> > the boot sequence: dom0 would still boot from xen_start_kernel,
> > everything else would be pretty much the same.
> 
> Ah, so not HVM exactly (you would only use the EPT/NPT/RV1/HAP for
> pagetables).. and PV for startup, spinlock, timers, debug, CPU, and
> backends. Thought sticking in the HVM container in PV that Mukesh
> made work would also benefit.

Yes for startup, spinlock, timers and backends. I would use HVM for cpu
operations too (no need for pv_cpu_ops.write_gdt_entry anymore for
example).


> Or just come back to the idea of "real" HVM device driver domains
> and have the PV dom0 be a light one loading the rest. But the setup of
> it is just so complex.. And the PV dom0 needs to deal with the PCI backend
> xenstore, and able to comprehend ACPI _PRT... and then launch the "device
> driver" Dom0, which at its simplest form would have all of the devices
> passed in to it.
> 
> So four payloads: PV dom0, PV dom0 initrd, HVM dom0, HVM dom0 initrd :-)
> Ok, that is too cumbersome. Maybe ingest the PV dom0+initrd in the Xen
> hypervisor binary.. I should stop here.

The goal of splitting up dom0 into multiple management domain is surely
a worthy goal, no matter is the domains are PV or HVM or PV on HVM, but
yeah the setup is hard. I hope that the we'll be able to simplify it in
the near future, maybe after the switchover to the new qemu and seabios
is completed.

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