Re: [libvirt] Analysis of the effect of adding PCIe root ports
On 10/10/2016 05:27 AM, Andrea Bolognani wrote:
On Fri, 2016-10-07 at 12:11 -0400, Laine Stump wrote:
>> So instead of guaranteeing that there will always be an empty
>> slot available for hotplug during a single start/destroy
>> cycle of the guest, we would be guaranteeing that there will
>> be 3/4 empty slots available for either hotplug or coldplug
>> throughout the entire life of the guest.
>
> A better way to put it is that we guarantee there will be "N" (3 or 4 or
> whatever) slots available when the domain is originally defined. Once
> any change has been made, all bets are off.
Okay, your alternative reading matches with my understanding
of your proposal :)
>> Sounds like a pretty good compromise to me.
>>
>> The only problem I can think of is that there might be
>> management applications that add eg. a pcie-root in the XML
>> when first defining a guest, and after the change such guests
>> would get zero hotpluggable ports. Then again it's probably
>> okay to expect such management applications to add the
>> necessary number of pcie-root-ports themselves.
>
> Yeah, if they know enough to be adding a root-port, then they know
> enough to add extras.
Note that I wrote pcie-root, not pcie-root-port.
>> Maybe we could relax the wording on A) and ignore any
>> pci{,e}-root? Even though there is really no reason for
>> either a user or a management application to add them
>> explicitly when defining a guest, I feel like they might be
>> special enough to deserve an exception.
>
> I thought about that; I'm not sure. In the case of libguestfs, even if
> Rich added a pcie-root, I guess he would still be manually addressing
> his pci devices, so that would be clue enough that he knew what he was
> doing and didn't want any extra.
Yeah, I'm not sure about that either, I just felt like it
would be good to think about it. I don't really see problems
going one way or the other, and I guess your initial
proposal is slightly more strict, so we might as well go for
it and relax it later if a compelling use case is found.
I implemented this over the weekend, and it turns out that it's much
less disruptive to ignore whether the pcie-root controller was added by
the user or by libvirt. This is because pcie-root is added much earlier
(when we are adding the implicit/default devices for the domain), so by
the time we get to the function that assigns PCI addresses (which is
where we auto-add all the additional PCI controllers) we no longer have
any information about whether pcie-root was specified in the XML, or if
it was auto-added.
The way I have it implemented now is to add the extra controllers if the
initial controller count was <= 1 *and* any new PCI controllers were
auto-added during address assignment (which implies that an unaddressed
device needed an address, so we added a controller to satisfy it). I
suppose if I want to be pedantic, I need to actually keep track of
whether or not there were any unaddressed devices (in the case that the
only devices they requested were primary video + USB2 + SATA controller,
no new PCI controllers would be added).