ron minnich wrote: > We had hoped to get something like this into Xen. On Xen, for example, > the block device and ethernet device interfaces are as different as > one could imagine. Disk I/O does not steal pages from the guest. The > network does. Disk I/O is in 4k chunks, period, with a bitmap > describing which of the 8 512-byte subunits are being sent. The enet > device, on read, returns a page with your packet, but also potentially > containing bits of other domain's packets too. The interfaces are as > dissimilar as they can be, and I see no reason for such a huge > variance between what are basically read/write devices. >
The reason for the variance is that hardware capabilities are very different for disk and block. Block device requests are always guest-initiated and sector-aligned, and often span many pages. On the other hand, network packets are byte aligned, and rx packets are host-initiated, triggering the stolen pages concept (which unsurprisingly turned out not to be a win). Network has such esoteric features as TSO. Block is very interested in actually getting things onto the disk (barrier support). In short, the "everything is a stream of bytes" grossly oversimplifies things. > Another issue is that kvm, in its current form (-24) is beautifully > simple. These additions seem to detract from the beauty a bit. Might > it be worth taking a little time to consider these ideas in order to > preserve the basic elegance of KVM? > kvm? elegant and simple? it's basically a pile of special cases. But I agree that the growing code base is a problem. With the block driver we can probably keep the host side in userspace, but to do the same for networking is much more work. I do think (now) that it is doable. -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
