Re: [PATCH v3 2/2] virtio-fs: add notification queue

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Jun 23, 2021 at 09:28:43AM -0400, Vivek Goyal wrote:
> On Wed, Jun 09, 2021 at 04:37:40PM +0100, Stefan Hajnoczi wrote:
> 
> [..]
> >  \subsection{Device configuration layout}\label{sec:Device Types / File System Device / Device configuration layout}
> >  
> > -All fields of this configuration are always available.
> > -
> >  \begin{lstlisting}
> >  struct virtio_fs_config {
> >          char tag[36];
> >          le32 num_request_queues;
> > +        le32 notify_buf_size;
> 
> Hi Stefan,
> 
> I am wondering if there should be a negotiation for number of descriptors
> in the notification queue between device and driver. How does it work
> for regular queus. I remeber there is a qemu has "queue-size" parameter
> which controls this.
> 
> But given this is notification queue and number of descriptors will be
> allocated by driver. For now I have hardcoded it to 16 in driver. But
> may be device should be able to specify how many descriptors to allocate
> for notification queue, possibly using configuration space? WDYT.

VIRTIO devices report the maximum supported queue size via
transport-specific means (e.g. virtio-pci's struct
virtio_pci_common_cfg->queue_size field). QEMU's -device
virtio-*,queue-size= sets this field.

The driver can reduce the queue size from that maximum, if desired.

Since this mechanism is already present I don't think anything
additional is needed in the virtiofs device specification.

Stefan

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