On Sun, 2010-06-20 at 21:59 +1000, Herbert Xu wrote: > On Sun, Jun 20, 2010 at 02:47:19PM +0300, Michael S. Tsirkin wrote: > > > > Let's do this then. So far the virtio spec avoided making layout > > assumptions, leaving guests lay out data as they see fit. > > Isn't it possible to keep supporting this with zero copy for hardware > > that can issue DMA at arbitrary addresses? > > I think you're mistaken with respect to what is being proposed. > Raising 512 bytes isn't a hard constraint, it is merely an > optimisation for Intel NICs because their PS mode can produce > a head fragment of up to 512 bytes. > > If the guest didn't allocate 512 bytes it wouldn't be the end of > the world, it'd just mean that we'd either copy whatever is in > the head fragment, or we waste 4096-X bytes of memory where X > is the number of bytes in the head. If I understand correctly what this 'PS mode' is (I haven't seen the documentation for it), it is a feature that Microsoft requested from hardware vendors for use in Hyper-V. As a result, the SFC9000 family and presumably other controllers also implement something similar. Ben. -- Ben Hutchings, Senior Software Engineer, Solarflare Communications Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- 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
