On 01/20/2011 10:07 AM, Michael S. Tsirkin wrote:
On Thu, Jan 20, 2011 at 09:43:57AM -0600, Anthony Liguori wrote:
On 01/20/2011 09:35 AM, Michael S. Tsirkin wrote:
When MSI is off, each interrupt needs to be bounced through the io
thread when it's set/cleared, so vhost-net causes more context switches and
higher CPU utilization than userspace virtio which handles networking in
the same thread.
We'll need to fix this by adding level irq support in kvm irqfd,
for now disable vhost-net in these configurations.
Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
I actually think this should be a terminal error. The user asks for
vhost-net, if we cannot enable it, we should exit.
Or we should warn the user that they should expect bad performance.
Silently doing something that the user has explicitly asked us not
to do is not a good behavior.
Regards,
Anthony Liguori
The issue is that user has no control of the guest, and can not know
whether the guest enables MSI. So what you ask for will just make
some guests fail, and others fail sometimes.
The user also has no way to know that version X of kvm does not expose a
way to inject level interrupts with irqfd.
We could have *another* flag that says "use vhost where it helps" but
then I think this is what everyone wants to do, anyway, and libvirt
already sets vhost=on so I prefer redefining the meaning of an existing
flag.
In the very least, there needs to be a vhost=force.
Having some sort of friendly default policy is fine but we need to
provide a mechanism for a user to have the final say. If you want to
redefine vhost=on to really mean, use the friendly default, that's fine
by me, but only if the vhost=force option exists.
I actually would think libvirt would want to use vhost=force. Debugging
with vhost=on is going to be a royal pain in the ass if a user reports
bad performance. Given the libvirt XML, you can't actually tell from
the guest and the XML whether or not vhost was actually in use or not.
Regards,
Anthony Liguori
Maybe this is best handled by a documentation update?
We always said:
" use vhost=on to enable experimental in kernel accelerator\n"
note 'enable' not 'require'. This is similar to how we specify
nvectors : you can not make guest use the feature.
How about this:
diff --git a/qemu-options.hx b/qemu-options.hx
index 898561d..3c937c1 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1061,6 +1061,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
" use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
" use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
" use vhost=on to enable experimental in kernel accelerator\n"
+ " (note: vhost=on has no effect unless guest uses MSI-X)\n"
" use 'vhostfd=h' to connect to an already opened vhost net device\n"
#endif
"-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"
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