On Sat, 2007-09-15 at 11:05 +0300, Avi Kivity wrote: > Rusty Russell wrote: > > On Sat, 2007-09-15 at 01:31 +0300, Dor Laor wrote: > > > >> Second, regardless of the channel signal notification, there are still > >> real necessities for userspace hypercall handling: > >> 1. For virtio drivers there is also registration hypercall for passing > >> the shared memory pfns. > >> Sure there are other possibilities, but why limit ourselves? > >> > > > > I really prefer doing this the more "hardware-like" way and having the > > device description say where the pages are. Surely this is simpler from > > the qemu side, too? > > > > This is mmio style (device provides memory). DMA style (guest provides > memory, device dmas it) is easier for kvm: we don't need to allocate a > new memory slot, and migration is easier. > > Most current hardware place the descriptor ring in guest memory, not the > device, since mmio is slower for the cpu than RAM.
But for virtual devices it seems unnecessarily convoluted: we'd have some mmio space (or PCI config space?) to tell the device where the ring buffer is. That's a little nasty for lguest early printk, too. I'm looking now at getting lguest to present a PCI bus implementation (probably changing virtbus to a series of PCI id plus bytes for "config space"). That's easier with a "device mem" model, since no config info needs to go back to the host. OTOH, with the guest memory model, guest can potentially choose size of ring. I don't know if that's an actual advantage (but I have code now which makes the ring size variable). Rusty. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
