On Mon, May 03, 2010 at 08:39:08AM -0700, David Stevens wrote: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote on 05/03/2010 03:34:11 AM: > > > On Wed, Apr 28, 2010 at 01:57:12PM -0700, David L Stevens wrote: > > > This patch adds mergeable receive buffer support to vhost_net. > > > > > > Signed-off-by: David L Stevens <dlstevens@xxxxxxxxxx> > > > > I've been doing some more testing before sending out a pull > > request, and I see a drastic performance degradation in guest to host > > traffic when this is applied but mergeable buffers are not in used > > by userspace (existing qemu-kvm userspace). > > Actually, I wouldn't expect it to work at all; > the qemu-kvm > patch (particularly the feature bit setting bug fix) is required. Which bugfix is that? > Without it, I think the existing code will tell the guest to use > mergeable buffers while turning it off in vhost. It should not do that, specifically vhost_net_get_features does: features &= ~(1 << VIRTIO_NET_F_MRG_RXBUF); unconditionally. This was added with: 5751995a20e77cd9d61d00f7390401895fa172a6 I forced mergeable buffers off with -global virtio-net-pci.mrg_rxbuf=off and it did not seem to help either. > That was completely > non-functional for me -- what version of qemu-kvm are you using? 992cc816c433332f2e93db033919a9ddbfcd1da4 or later should work well AFAIK. > What I did to test w/o mergeable buffers is turn off the > bit in VHOST_FEATURES. It should be enough to force mergeable buffers to off by qemu command line: -global virtio-net-pci.mrg_rxbuf=off > I'll recheck these, but qemu-kvm definitely > must be updated; the original doesn't correctly handle feature bits. > > +-DLS Hmm, I don't see the bug. -- MST -- 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
