Re: [Xen-devel] [PATCH 8/8] xen/pvh: Enable CPU hotplug

[Boris Ostrovsky]

On 10/27/2016 11:00 AM, Andrew Cooper wrote:
> On 27/10/16 15:25, Boris Ostrovsky wrote:
> > On 10/14/2016 03:01 PM, Boris Ostrovsky wrote:
> > > On 10/14/2016 02:41 PM, Andrew Cooper wrote:
> > > > On 14/10/16 19:05, Boris Ostrovsky wrote:
> > > > > PVH guests don't receive ACPI hotplug interrupts and therefore
> > > > > need to monitor xenstore for CPU hotplug event.
> > > > Why not?  If they don't, they should.  As we are providing ACPI anyway,
> > > > we should provide all bits of it.
> > >
> > > We don't have IOAPIC, which is how these interrupts are typically
> > > delivered. I suppose we might be able to specify it as something else.
> > >
> > > I'll look into this.
> >
> >
> > (+Jan)
> >
> > Yes, we can do this. The main issue is how to deal with event
> > registers (i.e FADT.x_pm1a_evt_blk) and AML's PRST region (which
> > specifies online CPU map).
> >
> > Currently these are accessed via IO space and are handled by qemu.
> >
> > There are a couple of ways to deal with this that I can see.
> >
> > 1. We can implement ioreq handling in the hypervisor, there are only a
> > few addresses that need handling.
> >
> > 2. We can implement those registers in memory space and have libxl
> > update them them on a hotplug command. This appears to be possible
> > because these registers mostly just consume writes without side
> > effects so they can be simple memory locations. The one exception is
> > updating status bits (they are cleared by writing 1s) but I think we
> > can do this from the AML.
> >
> > Other than that the only other thing is setting up an event channel
> > between the toolstack and the guest (either via xenstore or perhaps by
> > having a reserved port for SCI).
> >
> > I have a prototype with (2) (except for the bit clearing part) but I
> > want to hear comments on this approach before I write proper patches.
>
> Xen already deals with 1 for HVM guests.  We should do the same for PVH
> guests as well.

OK. The reason I was a little hesitant to go that route was because I 
didn't want to add more code to hypervisor.

But this is quite simpler than (2) --- we can keep all ACPI data common 
with HVM.

> -1 to anything involving looping a PVH dom0 back around to some entity
> running inside dom0.


This hotplug path would only be used by an unprivileged domain (i.e. the 
one whose ACPI data is created by the toolstack). But TBH I haven't 
thought about dom0 at all. And for that (1) is probably the only option.

-boris