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

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

[Nirmalanand J]

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Hello experts,

Have a query on the following:

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.

2) In case it is not negotiable with the back-end and unable to allocate
data buffers on the Rx side for 4096 descriptors, is there a workaround to
manage with fewer buffers like 256?

Additional info:
This is a FreeBSD VM running on GCP.

Please let me know.

Regards,
Nirmal

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

[Stefan Hajnoczi]

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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 4.1.4.8 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 (4.1.4.3
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
luck.

Stefan

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

[Nirmalanand J]

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Thanks Stefan. Appreciate your detailed response.

Regards,
Nirmal

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 4.1.4.8 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 (4.1.4.3
> 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
> luck.
>
> Stefan
>

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