Re: [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the maximum number of virtqueues to 513

[Michael S. Tsirkin]

On Tue, Apr 07, 2015 at 08:06:14PM +0200, 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.

Well, that's the # of VCPUs QEMU supports ATM, no?
Seems silly to have limit on vqs block us from creating
such VMs.

Maybe just define it as max cpus * 2 + 1.


> On the other hand, the message is dated april first...
> cheers
> luigi