"Michael S. Tsirkin" [off-list ref] wrote on 05/03/2010 03:34:11 AM:

On Wed, Apr 28, 2010 at 01:57:12PM -0700, David L Stevens wrote:

quoted

This patch adds mergeable receive buffer support to vhost_net.

Signed-off-by: David L Stevens <redacted>

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.
Without it, I think the existing code will tell the guest to use
mergeable buffers while turning it off in vhost. That was completely
non-functional for me -- what version of qemu-kvm are you using?
        What I did to test w/o mergeable buffers is turn off the
bit in VHOST_FEATURES. I'll recheck these, but qemu-kvm definitely
must be updated; the original doesn't correctly handle feature bits.

                                                                +-DLS