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

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Date:   Thu, 27 Oct 2016 10:25:13 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>, david.vrabel@...rix.com,
        JGross@...e.com
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        roger.pau@...rix.com, Jan Beulich <jbeulich@...e.com>
Subject: Re: [Xen-devel] [PATCH 8/8] xen/pvh: Enable CPU hotplug

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.

-boris