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

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: 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: