[RFC 1/2] virtio: Add AMBA bus driver for virtio device
Pawel Moll
pawel.moll at arm.com
Wed Sep 14 12:47:23 EDT 2011
> > 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?
>
> The former is almost certainly sufficient (remember, we can abuse the
> final feature bit to indicate there are extended features somewhere
> else, if we really have to). But agree the latter is preferable: this
> isn't time-critical.
Ok, I'll do this in v3 then.
> > > 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?
>
> Does Host care? It is sufficient for it to publish min/max values?
Hm. You're right, it should no care. Most likely the queue size will
impact some Host's memory allocation size, but that's the Host's
problem :-)
At the same time, how the Host would know what the max value is?
Generally it looks like the matter was getting back to value suggested
by the Host (as it is now) that may be changed by the Guest. Single R/W
register should be fine, I believe?
> You might want to throw the alignment value in there; there has been
> talk about adding that to PCI, for really small rings (less than a
> page).
Sure thing, will do.
v3 will be ready tomorrow.
Cheers!
Paweł
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