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Re: [Qemu-ppc] [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the ma


From: Jason Wang
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the maximum number of virtqueues to 513
Date: Wed, 08 Apr 2015 16:29:22 +0800



On Wed, Apr 8, 2015 at 2:33 AM, Alexander Graf <address@hidden> wrote:
On 04/07/2015 08:06 PM, Luigi Rizzo wrote:


On Tue, Apr 7, 2015 at 6:54 PM, Alexander Graf <address@hidden> wrote:
On 04/01/2015 10:15 AM, Jason Wang wrote:
This patch increases the maximum number of virtqueues for pci from 64 to 513. This will allow booting a virtio-net-pci device with 256 queue
pairs.
... * configuration space */ #define VIRTIO_PCI_CONFIG_SIZE(dev) VIRTIO_PCI_CONFIG_OFF(msix_enabled(dev))
  -#define VIRTIO_PCI_QUEUE_MAX 64
+#define VIRTIO_PCI_QUEUE_MAX 513

513 is an interesting number. Any particular reason for it? Maybe this was mentioned before and I just missed it ;)


quite large, too. I thought multiple queue pairs were useful
to split the load for multicore machines, but targeting VMs with
up to 256 cores (and presumably an equal number in the host)
seems really forward looking.

They can also be useful in case your host tap queue is full, so going higher than the host core count may make sense for throughput.

However, I am in doubt that there is a one-size-fits-all answer to this. Could we maybe make the queue size configurable via a qdev property?

We can do this on top but I'm not sure I understand the question. Do you mean a per-device limitation?



Alex





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