On Tue, 28 Apr 2015 14:47:11 +0200 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Tue, Apr 28, 2015 at 01:39:51PM +0200, Cornelia Huck wrote: > > On Tue, 28 Apr 2015 12:55:40 +0200 > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > On Tue, Apr 28, 2015 at 12:40:07PM +0200, Cornelia Huck wrote: > > > > On Tue, 28 Apr 2015 10:16:04 +0200 > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > > > > On Tue, Apr 28, 2015 at 10:04:15AM +0200, Cornelia Huck wrote: > > > > > > On Tue, 28 Apr 2015 09:14:07 +0200 > > > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > > > > > > > > On Tue, Apr 28, 2015 at 02:13:20PM +0800, Jason Wang wrote: > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Apr 28, 2015 at 1:13 PM, Michael S. Tsirkin > > > > > > > > <m...@redhat.com> wrote: > > > > > > > > >On Tue, Apr 28, 2015 at 11:14:04AM +0800, Jason Wang wrote: > > > > > > > > >> On Mon, Apr 27, 2015 at 7:05 PM, Michael S. Tsirkin > > > > > > > > >><m...@redhat.com> wrote: > > > > > > > > >> >On Thu, Apr 23, 2015 at 02:21:41PM +0800, Jason Wang wrote: > > > > > > > > >> >> This patch introduces a bus specific queue limitation. It > > > > > > > > >> >> will be > > > > > > > > >> >> useful for increasing the limit for one of the bus without > > > > > > > > >>disturbing > > > > > > > > >> >> other buses. > > > > > > > > >> >> Cc: Michael S. Tsirkin <m...@redhat.com> > > > > > > > > >> >> Cc: Alexander Graf <ag...@suse.de> > > > > > > > > >> >> Cc: Richard Henderson <r...@twiddle.net> > > > > > > > > >> >> Cc: Cornelia Huck <cornelia.h...@de.ibm.com> > > > > > > > > >> >> Cc: Christian Borntraeger <borntrae...@de.ibm.com> > > > > > > > > >> >> Cc: Paolo Bonzini <pbonz...@redhat.com> > > > > > > > > >> >> Signed-off-by: Jason Wang <jasow...@redhat.com> > > > > > > > > >> >> Reviewed-by: Cornelia Huck <cornelia.h...@de.ibm.com> > > > > > > > > >> > > > > > > > > > >> >Is this still needed if you drop the attempt to > > > > > > > > >> >keep the limit around for old machine types? > > > > > > > > >> If we agree to drop, we probably need transport specific > > > > > > > > >> macro. > > > > > > > > > > > > > > > > > >You mean just rename VIRTIO_PCI_QUEUE_MAX to VIRTIO_QUEUE_MAX? > > > > > > > > >Fine, why not. > > > > > > > > > > > > > > > > I mean keeping VIRTIO_PCI_QUEUE_MAX for pci only and just > > > > > > > > increase pci > > > > > > > > limit. And introduce e.g VIRTIO_PCI_QUEUE_CCW for ccw and keep > > > > > > > > it as 64. > > > > > > > > Since to my understanding, it's not safe to increase the limit > > > > > > > > for all other > > > > > > > > transports which was pointed out by Cornelia in V1: > > > > > > > > http://permalink.gmane.org/gmane.comp.emulators.qemu/318245. > > > > > > > > > > > > > > I think all you need is add a check to CCW_CMD_SET_IND: > > > > > > > limit to 64 for legacy interrupts only. > > > > > > > > > > > > It isn't that easy. > > > > > > > > > > > > What is easy is to add a check to the guest driver that fails setup > > > > > > for > > > > > > devices with more than 64 queues not using adapter interrupts. > > > > > > > > > > > > On the host side, we're lacking information when interpreting > > > > > > CCW_CMD_SET_IND (the command does not contain a queue count, and the > > > > > > actual number of virtqueues is not readily available.) > > > > > > > > > > Why isn't it available? All devices call virtio_add_queue > > > > > as appropriate. Just fail legacy adaptors. > > > > > > > > Because we don't know what the guest is going to use? It is free to > > > > use per-subchannel indicators, even if it is operating in virtio-1 mode. > > > > > > > > > > > We also can't > > > > > > fence off when setting up the vqs, as this happens before we know > > > > > > which > > > > > > kind of indicators the guest wants to use. > > > > > > > > > > > > More importantly, we haven't even speced what we want to do in this > > > > > > case. Do we want to reject SET_IND for devices with more than 64 > > > > > > queues? (Probably yes.) > > > > > > > > > > > > All this involves more work, and I'd prefer to do Jason's changes > > > > > > instead as this gives us some more time to figure this out properly. > > > > > > > > > > > > And we haven't even considered s390-virtio yet, which I really want > > > > > > to > > > > > > touch as little as possible :) > > > > > > > > > > Well this patch does touch it anyway :) > > > > > > > > But only small, self-evident changes. > > > > > > > > > > Sorry, I don't see what you are trying to say. > > > There's no chance legacy interrupts work with > 64 queues. > > > Guests should have validated the # of queues, and not > > > attempted to use >64 queues. Looks like there's no > > > such validation in guest, right? > > > > I have no idea whether > 64 queues would work with s390-virtio - it > > might well work, but I'm not willing to extend any effort to verifying > > that. > > Well this doesn't mean we won't make any changes, ever, > just so we can reduce verification costs. > Let's make the change everywhere, if we see issues > we'll backtrack. I don't like possibly breaking things with a seeing eye. And I know that some virtio-ccw setups will break. > > > > > > > Solution - don't specify this configuration with legacy guests. > > > > > > Modern guests work so there's value in supporting such > > > configuration in QEMU, I don't see why we must deny it in QEMU. > > > > What is "legacy guest" in your context? A guest running with the legacy > > transport or a guest using ccw but not virtio-1? A ccw guest using > > adapter interrupts but not virtio-1 should be fine. > > A guest not using adapter interrupts. There's nothing about that that's per-guest. It is a choice per-device. In fact, the Linux guest driver falls back to classic interrupts if it fails to setup adapter interrupts for a device - and this might happen for large guests when the host adapter routing table is full. > > > > > > > > > For s390 just check and fail at init if you like. > > > > > > > > What about devices that may change their number of queues? I'd really > > > > prefer large queue numbers to be fenced off in the the individual > > > > devices, and for that they need to be able to grab a transport-specific > > > > queue limit. > > > > > > This is why I don't want bus specific limits in core, > > > it just makes it too easy to sweep dirt under the carpet. > > > s390 is legacy - fine, but don't perpetuate the issue > > > in devices. > > > > What is "swept under the carpet" here? A device can have min(max queues > > from transport, max queues from device type) queues. I think it's > > easier to refuse instantiating with too many queues per device type (as > > most will be fine with 64 queues), so I don't want that code in the > > transport (beyond making the limit available). > > > > For s390 I'd like in the end: > > - s390-virtio: legacy - keep it working as best-can-do, so I'd prefer > > to keep it at 64 queues, even if more might work > > - virtio-ccw, devices in legacy or virtio-1 mode: works with adapter > > interrupts, so let's fence off setting per-subchannel indicators if a > > device has more than 64 queues (needs work and a well thought-out > > rejection mechanism) > > > > That's _in the end_: I'd like to keep ccw at 64 queues _for now_ so > > that we don't have a rushed interface change - and at the same time, I > > don't want to hold off pci. Makes sense? > > If you want to fail configurations with > 64 queues in ccw or s390, > that's fine by me. I don't want work arounds for these bugs in virtio > core though. So transports should not have a say in how many queues can > be supported, but they can fail configurations they can't support if > they want to. Eh, isn't that a contradiction? Failing a configuration means that the transport does indeed have a say?
