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Date: Wed, 8 Apr 2015 15:23:12 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the maximum
 number of virtqueues to 513
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To: Luigi Rizzo <rizzo@iet.unipi.it>
Cc: Jason Wang <jasowang@redhat.com>, Alexander Graf <agraf@suse.de>, qemu-devel <qemu-devel@nongnu.org>, qemu-ppc@nongnu.org, cornelia.huck@de.ibm.com, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

On Tue, Apr 07, 2015 at 08:06:14PM +0200, Luigi Rizzo wrote:
>=20
>=20
> On Tue, Apr 7, 2015 at 6:54 PM, Alexander Graf <agraf@suse.de> wrote:
>=20
>     On 04/01/2015 10:15 AM, Jason Wang wrote:
>=20
>         This patch increases the maximum number of virtqueues for pci f=
rom 64
>         to 513. This will allow booting a virtio-net-pci device with 25=
6 queue
>         pairs.
>         ...
>=20
>         =A0 =A0* configuration space */
>         =A0 #define VIRTIO_PCI_CONFIG_SIZE(dev)=A0 =A0 =A0VIRTIO_PCI_CO=
NFIG_OFF
>         (msix_enabled(dev))
>         =A0 -#define VIRTIO_PCI_QUEUE_MAX 64
>         +#define VIRTIO_PCI_QUEUE_MAX 513
>=20
>=20
>     513 is an interesting number. Any particular reason for it? Maybe t=
his was
>     mentioned before and I just missed it ;)
>=20
>=20
>=20
> 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