Re: [Qemu-devel] [PATCH V4 00/19] Support more virtio queues

[Michael S. Tsirkin]

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 <address@hidden> 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
> >><address@hidden> 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