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SubjectRe: [RFC 1/2] virtio: Add AMBA bus driver for virtio device
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> > No problem, I've reserved 128 bits in the modified registers layout (see
> > RFC v2), I can even change it into "offset/value" interface, which would
> > give you variable width. One thing I don't get is how to access feature
> > bits > 31? The get_features() seems
> > to be limited to 32:
> >
> > struct virtio_config_ops {
> > <...>
> > u32 (*get_features)(struct virtio_device *vdev);
> > <...>
> > }
>
> There's a patch for that already :)

Ok, so how about one of the following two options:

1. 4 * 32-bit words of feature flags, 128 bits in total:

* 0x010 32 HostFeatures0 Features supported by the host, bits 0-31
* 0x014 32 HostFeatures1 Features supported by the host, bits 32-63
* 0x018 32 HostFeatures2 Features supported by the host, bits 64-95
* 0x01c 32 HostFeatures3 Features supported by the host, bits 96-127

* 0x020 32 GuestFeatures0 Features activated by the guest, bits 0-31
* 0x024 32 GuestFeatures1 Features activated by the guest, bits 32-63
* 0x028 32 GuestFeatures2 Features activated by the guest, bits 64-95
* 0x02c 32 GuestFeatures3 Features activated by the guest, bits 96-127

2. "Select word -> read flags" interface:

* 0x010 32 HostFeaturesFlags Features supported by the host
* 0x014 32 HostFeaturesWord Set of features to access

* 0x020 32 GuestFeaturesFlags Features activated by the guest
* 0x024 32 GuestFeaturesWord Set of features to set

In this case the algorithm would be:

writel(0, HostFeaturesWord);
bits_0_31 = readl(HostFeaturesFlags);
writel(1, HostFeaturesWord);
bits_32_63 = readl(HostFeaturesFlags);
<etc>

The latter seems more "future-proof" to me, if slightly more complex...
Any other suggestions?

> > > > + * 0x008 32 QueuePFN PFN for the currently selected queue
> > > > + * 0x00c 32 QueueNum Queue size for the currently selected queue
> > >
> > > You should, I believe, seriously consider allowing the guest to set the
> > > queue size, rather than the host (perhaps the host could suggest one,
> > > but the guest should be able to override it).
> >
> > I guess the QueueNum could be simply a read/write register - guest can
> > read suggestion and override it writing it back. The same question
> > though - how can guest change it? (I mean some API?)
>
> We can sort that out later... the Guest may try some ambitious large
> allocation and fall back if it fails.

So, to my mind, the negotiations could look like this (from the Guest's
point of view):

1. (optional) The Guest is asking what size is "suggested" by the Host:

size = readl(QueueNum);

2. The Guest requests size it would like to use:

writel(optimal_size(size), QueueNum);

3. The Host does the best it can to accommodate the Guest and the Guest
checks the effective size:

size = real(QueueNum);

Does it make some sense?

> > > Anthony or Michael might suggest other changes, since they are most
> > > familiar with virtio_pci limitations...
> >
> > I've added them on the RFC v2 Cc as well - all feedback more then
> > welcome!
>
> Excellent... of course, the virtualization list is down at the moment,
> making this a bit more awkward.

Haha - at least once I can't blame our corporate IT for making my job
harder ;-)

Cheers!

Paweł

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