Messages in this thread | | | Date | Sun, 6 Nov 2011 09:30:07 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: virtio-pci new configuration proposal |
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On Fri, Nov 04, 2011 at 04:53:05PM +0200, Sasha Levin wrote: > > > > > As you said, the PCI cap list was introduced both to save space (which > > > > > is not the motivation here), and because it's a very efficient > > > > > > > > It's actually pretty inefficient - there's an overhead of 3 bytes for > > > > each vendor specific option. > > > > > > It's efficient because while you pay a small price for each optional > > > option it also means that that option is optional and won't clutter the > > > config space if it's not really in use. > > > > I guess my assumption is that most options will be in use, > > not discarded dead-ends. > > I don't know about that. 64 bit features would be pretty rare for now - > and I don't think that setting the alignment will be also enabled by > default.
Setting the alignment might not be *used* by default but I think it must be enabled by default to allow bios access.
> I think that we're looking at it differently because I assume that any > feature we add at this point would be optional and used only in specific > scenarios, while you think that everything added will be used most of > the time.
Options must often be present even if not used. For example, as device has no way to know whether a guest will want to program alignment, it has to make that option available.
-- MST
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