Messages in this thread | | | Subject | Re: [RFC 1/2] virtio: Add AMBA bus driver for virtio device | From | Pawel Moll <> | Date | Tue, 13 Sep 2011 10:40:32 +0100 |
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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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