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Subject: Re: [virtio-dev] Handling large VIRTIO_PCI_QUEUE_NUM values on Google Cloud Platform (GCP)

Thanks Stefan. Appreciate your detailed response.


On Fri, 27 Apr 2018, 5:52 pm Stefan Hajnoczi, <stefanha@redhat.com> wrote:
On Wed, Apr 25, 2018 at 04:04:37PM +0530, Nirmalanand J wrote:
> 1) The number of descriptor count returned by the back-end through
> VIRTIO_PCI_QUEUE_NUM, is that negotiable with the backend? In some
> platforms like Google Cloud Platform (GCP) the value returned is 4096.

VIRTIO_PCI_QUEUE_NUM is the legacy VIRTIO PCI hardware register
(called "queue_size" in Legacy Interfaces: A Note on PCI Device
Layout).  This field is read-only.

However, in VIRTIO 1.0 the modern VIRTIO PCI queue_size field (
Common configuration structure layout) is writable during device
initialization.  The driver may *reduce* the queue size.

If you would like to use 256 descriptors instead of 4096, please make
sure your driver uses VIRTIO 1.0 (VIRTIO_F_VERSION_1) and not the legacy
interface.  Then the driver can reduce the number of descriptors to 256.

If the device does not support VIRTIO_F_VERSION_1 then you are out of


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