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Date: Thu, 19 Mar 2015 15:42:56 +0800
From: Jason Wang <jasowang@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH V4 00/19] Support more virtio queues
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, cornelia.huck@de.ibm.com, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, Keith Busch <keith.busch@intel.com>, Christian Borntraeger <borntraeger@de.ibm.com>, qemu-ppc@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Amit Shah <amit.shah@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>



On Thu, Mar 19, 2015 at 3:32 PM, Michael S. Tsirkin <mst@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 
>> <mst@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.

Another reason is not wasting memory for the extra virtqueues allocated 
for legacy machine types.