On Mon, Sep 26, 2011 at 12:04:47PM +0200, Alexander Graf wrote: > Am 26.09.2011 um 09:51 schrieb David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>: [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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
