Re: [virtio-dev] [PATCH v2 1/1] virtio-ism: introduce new device virtio-ism

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 11 Jan 2023 16:30:34 +0100, Halil Pasic <pasic@linux.ibm.com> wrote:
> On Wed, 11 Jan 2023 19:08:53 +0800
> Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> > > > +\begin{description}
> > > > +\item[\field{ev_type}] The type of event, the driver can get the size of the
> > > > +    structure based on this.
> > > > +
> > > > +\item[\field{offset}] The offset of ism regions with the event.
> > >
> > > Offset with respect to what?
> >
> > Used to specify a region. Offset is the position of this ISM Region inside the
> > memory of Device.
>
> An offset is per definition always relative to something. I would have
> thought this is an offset relative to the beginning of *the* virtio
> shared memory region identified by the ismid 1. But since you claim that
> there may be multiple virtio shared memory regions with the shmid 1 I'm
> heavily confused. What is here "the memory of Device"?

A device may indeed have multiple virtio shared memory regions. Although the
shmid of these virtio shared memory regions are all 1, there will be an order
when we initialize the device. For example, for PCI device we will check the PCI
cap one by one, so there is an order between these virtio shared memory regions.

This is described in the spec:
	+The shared memory whose shmid is VIRTIO_ISM_SHM_ID_REGIONS is used to implement
	+ism regions. If there are multiple shared memories whose shmid is
	+VIRTIO_ISM_SHM_ID_REGIONS, they are used as contiguous memory in the order of
	+acquisition.

Example:

	offset: 0                                                               1G - 1
	        |  virtio shared memory region (shmid: 1) (size: 1G) (order: 0) |

	offset: 1G                                                              3G - 1
	        |  virtio shared memory region (shmid: 1) (size: 2G) (order: 1) |

	offset: 3G                                                              7G - 1
	        |  virtio shared memory region (shmid: 1) (size: 4G) (order: 2) |

Thanks.



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