On Thu, Mar 19, 2015 at 03:42:56PM +0800, Jason Wang wrote: > > > On Thu, Mar 19, 2015 at 3:32 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > >On Thu, Mar 19, 2015 at 01:24:53PM +0800, Jason Wang wrote: > >> On Wed, Mar 18, 2015 at 8:58 PM, Michael S. Tsirkin > >><m...@redhat.com> wrote: > >> >On Wed, Mar 18, 2015 at 05:34:50PM +0800, Jason Wang wrote: > >> >> We current limit the max virtio queues to 64. This is not sufficient > >> >> to support multiqueue devices (e.g recent Linux support up to 256 > >> >> tap queues). So this series tries to let virtio to support more > >>queues. > >> >> No much works need to be done except: > >> >> - Introducing transport specific queue limitation. > >> >> - Let each virtio transport to use specific limit. > >> >> - Speedup the MSI-X masking and unmasking through per vector queue > >> >> list, and increase the maximum MSI-X vectors supported by qemu. > >> >> - With the above enhancements, increase the maximum number of > >> >> virtqueues supported by PCI from 64 to 513. > >> >> - Compat the changes for legacy machine types. > >> > > >> >What are the compatibility considerations here? > >> Two considerations: > >> 1) To keep msix bar size to 4K for legacy machine types > >> 2) Limit the pci queue max to 64 for legacy machine types > > > >2 seems not relevant to me. > > If we don't limit this. Consider migration from 2.4 to 2.3 > > Before migration > > write 0 to queue_sel > write 100 to queue_sel > read queue_sel will get 100 > > but after migration > > write 0 to queue_sel > write 100 to queue_sel > read queue_sel will get 0 > > The hardware behavior was changed after migration.
But this driver is out of spec - drivers are not supposed to select non-existent queues. So this doesn't matter. > Another reason is not wasting memory for the extra virtqueues allocated for > legacy machine types. If that's a significant amount of memory, we need to work to reduce memory consumption for new machine types. Not many people use the compat machine types, especially upstream. -- MST
