Re: [PATCH] virtio-blk: support polling I/O

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 14, 2022 at 12:25:08PM +0200, Max Gurtovoy wrote:
> 
> On 3/14/2022 11:43 AM, Suwan Kim wrote:
> > On Sun, Mar 13, 2022 at 12:37:21PM +0200, Max Gurtovoy wrote:
> > > On 3/11/2022 6:07 PM, Suwan Kim wrote:
> > > > On Fri, Mar 11, 2022 at 10:38:07AM -0500, Michael S. Tsirkin wrote:
> > > > > On Sat, Mar 12, 2022 at 12:28:32AM +0900, Suwan Kim wrote:
> > > > > > diff --git a/include/uapi/linux/virtio_blk.h b/include/uapi/linux/virtio_blk.h
> > > > > > index d888f013d9ff..3fcaf937afe1 100644
> > > > > > --- a/include/uapi/linux/virtio_blk.h
> > > > > > +++ b/include/uapi/linux/virtio_blk.h
> > > > > > @@ -119,8 +119,9 @@ struct virtio_blk_config {
> > > > > >    	 * deallocation of one or more of the sectors.
> > > > > >    	 */
> > > > > >    	__u8 write_zeroes_may_unmap;
> > > > > > +	__u8 unused1;
> > > > > > -	__u8 unused1[3];
> > > > > > +	__virtio16 num_poll_queues;
> > > > > >    } __attribute__((packed));
> > > > > Same as any virtio UAPI change, this has to go through the virtio TC.
> > > > > In particular I don't think gating a new config field on
> > > > > an existing feature flag is a good idea.
> > > > Did you mean that the polling should be based on a new feature like
> > > > "VIRTIO_BLK_F_POLL" and be added at the end of features_legacy[]
> > > > and features[]? If then, I will add the new feture flag and resend it.
> > > Isn't there a way in the SPEC today to create a queue without interrupt
> > > vector ?
> > It seems that it is not possible to create a queue without interrupt
> > vector. If it is possible, we can expect more polling improvement.

Yes, it's possible:

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to
\field{config_msix_vector}/\field{queue_msix_vector} maps interrupts triggered
by the configuration change/selected queue events respectively to
the corresponding MSI-X vector. To disable interrupts for an
event type, the driver unmaps this event by writing a special NO_VECTOR
value:

\begin{lstlisting}
/* Vector value used to disable MSI for queue */
#define VIRTIO_MSI_NO_VECTOR            0xffff
\end{lstlisting}



> MST/Jason/Stefan,
> 
> can you confirm that please ?
>
> what does VIRTQ_AVAIL_F_NO_INTERRUPT supposed to do ?

This is a hint to the device not to send interrupts.

> 
> > Regards,
> > Suwan Kim

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