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

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Date:   Thu, 27 Oct 2016 16:00:26 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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 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.

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

~Andrew

~Andrew