On Sun, 2012-08-12 at 11:36 +0300, Avi Kivity wrote: > On 08/09/2012 10:26 PM, Alex Williamson wrote: > > On Mon, 2012-08-06 at 13:40 +0300, Avi Kivity wrote: > >> On 08/06/2012 01:38 PM, Avi Kivity wrote: > >> > >> > Regarding the implementation, instead of a linked list, would an array > >> > of counters parallel to the bitmap make it simpler? > >> > >> Or even, replace the bitmap with an array of counters. > > > > I'm not sure a counter array is what we're really after. That gives us > > reference counting for the irq source IDs, but not the key->gsi lookup. > > You can look up the gsi while registering the eoifd, so it's accessible > as eoifd->gsi instead of eoifd->source->gsi. The irqfd can go away > while the eoifd is still active, but is this a problem? In my opinion, no, but Michael disagrees. > > It also highlights another issue, that we have a limited set of source > > IDs. Looks like we have BITS_PER_LONG IDs, with two already used, one > > for the shared userspace ID and another for the PIT. How happy are we > > going to be with a limit of 62 level interrupts in use at one time? > > When we start being unhappy we can increase that number. On the other > hand more locks and lists makes me unhappy now. Yep, good point. My latest version removes the source ID object lock and list (and objects). I still have a lock and list for the ack notification, but it's hard not to unless we combine them into one mega-irqfd ioctl as Michael suggests. > > It's arguably a reasonable number since the most virtualization friendly > > devices (sr-iov VFs) don't even support this kind of interrupt. It's > > also very wasteful allocating an entire source ID for a single GSI > > within that source ID. PCI supports interrupts A, B, C, and D, which, > > in the most optimal config, each go to different GSIs. So we could > > theoretically be more efficient in our use and allocation of irq source > > IDs if we tracked use by the source ID, gsi pair. > > There are, in one userspace, just three gsis available for PCI links, so > you're compressing the source id space by 3. I imagine there's a way to put each PCI interrupt pin on a GSI, but still only 4, not a great expansion of source ID space. I like Michael's idea of re-using source IDs if we run out better. > > That probably makes it less practical to replace anything at the top > > level with a counter array. The key that we pass back is currently the > > actual source ID, but we don't specify what it is, so we could split it > > and have it encode a 16bit source ID plus 16 bit GSI. It could also be > > an idr entry. > > We can fix those kinds of problems by adding another layer of > indirection. But I doubt they will be needed. I don't see people > assigning 60 legacy devices to one guest. Yep, we can ignore it for now and put it in the hands of userspace to re-use IDs if needed. > > Michael, would the interface be more acceptable to you if we added > > separate ioctls to allocate and free some representation of an irq > > source ID, gsi pair? For instance, an ioctl might return an idr entry > > for an irq source ID/gsi object which would then be passed as a > > parameter in struct kvm_irqfd and struct kvm_eoifd so that the object > > representing the source id/gsi isn't magically freed on it's own. This > > would also allow us to deassign/close one end and reconfigure it later. > > Thanks, > > Another option is to push the responsibility for allocating IDs for the > association to userspace. Let userspace both create the irqfd and the > eoifd with the same ID, the kernel matches them at registration time and > copies the gsi/sourceid from the first to the second eventfd. Aside from the copying gsi/sourceid bit, you've just described my latest attempt at this series. Specifying both a sourceid and gsi also allows userspace to make better use of the sourceid address space (use more than one gsi if userspace wants the complexity of managing them). Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
