Re: [virtio-dev] Re: [PATCH v1 2/2] virtio-mmio: add features for virtio-mmio specification version 3

[Jason Wang]

On 2020/1/2 äå5:13, Liu, Jing2 wrote:
> [...]
>
> > > > > +
> > > > > +/* RO: MSI feature enabled mask */
> > > > > +#define VIRTIO_MMIO_MSI_ENABLE_MASKÂÂÂ 0x8000
> > > > > +/* RO: Maximum queue size available */
> > > > > +#define VIRTIO_MMIO_MSI_STATUS_QMASKÂÂÂ 0x07ff
> > > > > +/* Reserved */
> > > > > +#define VIRTIO_MMIO_MSI_STATUS_RESERVEDÂÂÂ 0x7800
> > > > > +
> > > > > +#define VIRTIO_MMIO_MSI_CMD_UPDATEÂÂÂ 0x1
> > > >
> > > >
> > > > I believe we need a command to read the number of vectors supported 
> > > > by the device, or 2048 is assumed to be a fixed size here?
> > >
> > > For not bringing much complexity, we proposed vector per queue and 
> > > fixed relationship between events and vectors.
> >
> >
> > It's a about the number of MSIs not the mapping between queues to 
> > MSIs.And it looks to me it won't bring obvious complexity, just need 
> > a register to read the #MSIs. Device implementation may stick to a 
> > fixed size.
>
> Based on that assumption, the device supports #MSIs = #queues + 
> #config. Then driver need not read the register.
>
> We're trying to make such kind of agreement on spec level.


Ok, I get you now.

But still, having fixed number of MSIs is less flexible. E.g:

- for x86, processor can only deal with about 250 interrupt vectors.
- driver may choose to share MSI vectors [1] (which is not merged but we 
will for sure need it)

[1] https://lkml.org/lkml/2014/12/25/169


> > Having few pages for a device that only have one queue is kind of a 
> > waste.
>
> Could I ask what's the meaning of few pages here? BTW, we didn't 
> define MSIx-like tables for virtio-mmio.


I thought you're using a fixed size (2048) for each device. But looks not :)

Thanks


> Thanks,
>
> Jing
>
> > Thanks
> >
> >
> > > So the number of vectors supported by device is equal to the total 
> > > number of vqs and config.
> > >
> > > We will try to explicitly highlight this point in spec for later 
> > > version.
> > >
> > >
> > > Thanks!
> > >
> > > Jing
> > >
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