Re: [PATCHv3 RFC] virtio-pci: flexible configuration layout

[Sasha Levin <levinsasha928@gmail.com>]

On Wed, 2011-11-23 at 10:49 +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 23, 2011 at 01:02:22PM +1030, Rusty Russell wrote:
> > +/* Fields in VIRTIO_PCI_CAP_COMMON_CFG: */
> > +struct virtio_pci_common_cfg {
> > +	/* About the whole device. */
> > +	__u64 device_features;	/* read-only */
> > +	__u64 guest_features;	/* read-write */
> 
> We currently require atomic accesses to common fields.
> Some architectures might not have such for 64 bit,
> so these need to be split I think ...

We can consider stealing the feature implementation from virtio-mmio:
Use the first 32 bits as a selector and the last as the features
themselves.

It's more complex to work with, but it provides 2**32 32 feature bits
(which should be enough for a while) and it solves the atomic access
issue.

> > +	__u64 queue_address;	/* read-write */
> > +	__u16 msix_config;	/* read-write */
> > +	__u8 device_status;	/* read-write */
> > +	__u8 unused;
> > +
> > +	/* About a specific virtqueue. */
> > +	__u16 queue_select;	/* read-write */
> > +	__u16 queue_align;	/* read-write, power of 2. */
> > +	__u16 queue_size;	/* read-write, power of 2. */
> > +	__u16 queue_msix_vector;/* read-write */
> > +};
> >  #endif
> > 
> > 

-- 

Sasha.