[libvirt] Fwd: In Use tracker for network and pci-passthrough devices: Laine response

[Shradha Shah]

This is a reply I got from Laine Stump
=====================================================================================================================

(NB: I'm Cc'ing Osier on this email, as he's quite knowledgeable about
the PCI passthrough device allocation tracking code. You should probably
move this discussion to the mailing list sooner rather than later
though, as a public discussion of the design will give you a better
chance of your first revision getting successfully past review :-))

On 06/26/2012 07:23 AM, Shradha Shah wrote:
> > Laine,
> >
> > I have submitted my v2 patches for forward mode='hostdev' and am planning to work on the in-use tracker for network
> > and pci-passthrough devices.
> >
> > I am unable to wrap my head around how I should be implementing this functionality. I am unable to decide at what 
> > level I should be implementing this (network, domain or qemu).
> >
> > May I ask for your guidance in order to implement this functionality? 
> >
Yes, but I'm currently on vacation (in Turkey) so I won't have much time
to respond until July 9 when I return.

In the meantime, I think the right way to do this is by integrating with
the code in the qemu driver that keeps track of which PCI devices are in
use. This already happens at the very basic level of "if the device
allocated by the network driver is in use, the attempt to assign the
device will fail"; instead, the network driver should be able to ask
qemu if the device it wants to allocate to the guest is already in use
(and reserve it, in one atomic operation).

Of course, once the network driver has reserved the device from qemu's
PCI passthrough code, it would return that device to the qemu driver
code that wants to attach the interface, and it would fail because it
would be told the device is already in use (well, yeah! *We* just marked
it as in-use!). To make that work, I guess some sort of
cookie/handle/pointer would need to be passed from qemu's pci
passthrough code back to the network driver, and the network driver
would return it back to qemu's network interface attach code, which
would then use that special cookie/handle/pointer to attach the device
(saying "yeah, I know it's already in use, and here's my pass-card").

(Talking about this makes me think that the code that keeps track of PCI
device allocation shouldn't really be a part of qemu, but should be a
separate module, so that the network driver can still function properly
even if the qemu driver isn't loaded.)

Another twist to this that should be considered - if any particular
device is in use by at least one guest for one of the macvtap modes,
that device also needs to be marked as in-use in libvirt's pci device
table - it would be disastrous if another guest decided to use that
device for standard PCI Passthrough.

(Keep in mind that I wrote everything above without even once looking at
the code or any other reference, so you should take it with a grain of
salt!)



Many Thanks,
Regards,
Shradha Shah

On 06/28/2012 11:19 AM, Shradha Shah wrote:
> This is a conversation that I started with Laine Stump for the implementation of the in-use tracker for network and pci devices.
> 
> I want to make this conversation more public in order to receive everyone's view on the topic.
> 
> I will also post the responses I got from Laine and Osier Yang.
> 
> Many Thanks,
> Regards,
> Shradha Shah
> 
> 
> -------- Original Message --------
> Subject: In Use tracker for network and pci-passthrough devices
> Date: Tue, 26 Jun 2012 12:23:52 +0100
> From: Shradha Shah <sshah at solarflare.com>
> To: Laine Stump <laine at laine.org>
> 
> Laine,
> 
> I have submitted my v2 patches for forward mode='hostdev' and am planning to work on the in-use tracker for network
> and pci-passthrough devices.
> 
> I am unable to wrap my head around how I should be implementing this functionality. I am unable to decide at what 
> level I should be implementing this (network, domain or qemu).
> 
> May I ask for your guidance in order to implement this functionality? 
>