On Mon, Aug 13, 2012 at 04:41:05PM -0600, Alex Williamson wrote: > On Tue, 2012-08-14 at 01:06 +0300, Michael S. Tsirkin wrote: > > On Mon, Aug 13, 2012 at 03:34:01PM -0600, Alex Williamson wrote: > > > 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 > > > > Turns out per device source ID is a bug copied from existing > > device assignment. I am amazed we did not notice before. > > There we have small # of devices so it's not a problem but there's no > > reason just not to have a source ID for all irqfds. > > So the problem goes away, and there is no limit on # of level irqfds, > > and no need to manage IDs in userspace at all. > > You can still have cookies in userspace if you like but do not map them > > to source IDs. > > IMHO it's not a bug, it's an implementation decision. They could be > shared, but that doesn't make it wrong to not share them. Given that we > have 32 memory slots, the only way you could hit this would be to have a > lot of really slow devices that don't direct-map any BARs. A reason to > not have the same source id for everything is that I think we can do ack > notification filtering more easily using separate source ids (as is done > in the first patch of the v8 series). Just a thought: can filtering read and clear the irqfd counter? > As the code is today, I agree, > there's probably no advantage to using multiple source IDs. Thanks, > > Alex I think one point worth addressing is, Gleb wanted to get eoifd without irqfd at all and that works for timer interrupt. -- MST -- 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
