On Mon, Sep 26, 2011 at 12:04:47PM +0200, Alexander Graf wrote:
> Am 26.09.2011 um 09:51 schrieb David Gibson <[email protected]>:
[snip]
> > Um, not to put too fine a point on it, this is madness.
> >
> > Yes, it's very flexible and can thereby cover a very wide range of
> > cases. But it's much, much too complex. Userspace has to parse a
> > complex, multilayered data structure, with variable length elements
> > just to get an address at which to do IO. I can pretty much guarantee
> > that if we went with this, most userspace programs using this
> > interface would just ignore this metadata and directly map the
> > offsets at which they happen to know the kernel will put things for
> > the type of device they care about.
> >
> > _At least_ for PCI, I think the original VFIO layout of each BAR at a
> > fixed, well known offset is much better. Despite its limitations,
> > just advertising a "device type" ID which describes one of a few fixed
> > layouts would be preferable to this. I'm still hoping, that we can do
> > a bit better than that. But we should try really hard to at the very
> > least force the metadata into a simple array of resources each with a
> > fixed size record describing it, even if it means some space wastage
> > with occasionally-used fields. Anything more complex than that and
> > userspace is just never going to use it properly.
>
> We will have 2 different types of user space. One wants to be as
> generic as possible and needs all this dynamic information. QEMU
> would fall into this category.
>
> The other one is device specific drivers in user space. Here
> hardcoding might make sense.
>
> For the generic interface, we need something that us as verbose as
> possible and lets us enumerate all the device properties, so we can
> properly map and forward them to the guest.
>
> However, nothing keeps us from mapping BARs always at static offsets
> into the file. If you don't need the generic info, then you don't
> need it.
This sounds dangerous to me. I can just see some future kernel hacker
going "heey, Ican rearrange these, their locations are all advertised,
right".
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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