Re: [RFC 13/13] iommu/virtio: Add topology description to

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Fri, Nov 22, 2019 at 07:53:19AM -0500, Michael S. Tsirkin wrote:
> Overall this looks good to me. The only point is that
> I think the way the interface is designed makes writing
> the driver a bit too difficult. Idea: if instead we just
> have a length field and then an array of records
> (preferably unions so we don't need to work hard),
> we can shadow that into memory, then iterate over
> the unions.
> 
> Maybe add a uniform record length + number of records field.
> Then just skip types you do not know how to handle.
> This will also help make sure it's within bounds.
> 
> What do you think?

Sounds good, that should simplify the implementation a bit.

> You will need to do something to address the TODO I think.

Yes, I'll try to figure out a way to test platform devices.

> > +static void viommu_cwrite(struct pci_dev *dev, int cfg,
> > +			  struct viommu_cap_config *cap, u32 length, u32 offset,
> > +			  u32 val)
> 
> A single user with 4 byte parameter. Just open-code?

Ok

> > +		cap.head.type = viommu_cread(dev, pci_cfg, dev_cfg, 2, offset);
> > +		cap.head.next = viommu_cread(dev, pci_cfg, dev_cfg, 2, offset + 2);
> 
> All of this doesn't seem to be endian-clean. Try running sparse I think
> it will complain.

It does, I'll fix this

> > @@ -36,6 +37,31 @@ struct virtio_iommu_config {
> >  	struct virtio_iommu_range_32		domain_range;
> >  	/* Probe buffer size */
> >  	__le32					probe_size;
> > +	/* Offset to the beginning of the topology table */
> > +	__le16					topo_offset;
> 
> why do we need an offset?

I find it awkward to put a variable-size array in the middle of the
config. The virtio_iommu_config struct would be easier to extend later if
we keep the array at the end and only define small static fields here.

> 
> > +};
> > +
> > +struct virtio_iommu_topo_head {
> > +	__le16					type;
> > +	__le16					next;
> > +};
> 
> So this linked list makes things harder than necessary imho.
> It will be easier to just have a counter with # of records.
> Then make all records the same size.
> Then just read each record out into a buffer, and
> handle it there.

Yes, that should simplify things.

Thanks,
Jean