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Re: [libvirt] vhost-net configuration

On 05/20/2011 01:12 PM, Cal Heldenbrand wrote:

First look in /var/log/libvirt/qemu/<domainname>.log and see which qemu binary is being executed by libvirt - it may not be the one you've built.

It looks correct...  I pulled out the driver tag from my config and fired it up.  Here is the full command line for it:

LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/sbin:/usr/local/bin QEMU_AUDIO_DRV=none /usr/local/bin/qemu-system-x86_64 -S -M pc-0.14 -enable-kvm -m 4096 -smp 7,sockets=7,cores=1,threads=1 -name web101 -uuid bda63821-ccc4-6607-67c0-db6712526cbd -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/web101.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=readline -rtc base=utc -boot c -drive file=/var/lib/libvirt/images/web101.img,if=none,id=drive-virtio-disk0,format=raw,cache=writethrough -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0 -drive file=/home/httpd.img,if=none,id=drive-virtio-disk1,format=raw,cache=writethrough -device virtio-blk-pci,bus=pci.0,addr=0x7,drive=drive-virtio-disk1,id=virtio-disk1 -drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -fsdev local,security_model=passthrough,id=fsdev-fs0,path=/home/httpd -device virtio-9p-pci,id=fs0,fsdev=fsdev-fs0,mount_tag=/home/httpd,bus=pci.0,addr=0x8 -netdev tap,fd=15,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:6f:51:43,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -device usb-tablet,id=input0 -vnc -vga cirrus -device AC97,id=sound0,bus=pci.0,addr=0x4 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

Then, run "qemu-kvm --help" (replacing "qemu-kvm" with the path to the binary that libvirt is executing, found in the previous paragraph), and search through that output for the string ",vhost=" - if it's not there, then your qemu doesn't support vhost-net.

This also looks correct.  Is it wrong that my qemu compilation didn't create a binary named "qemu-kvm"?  (Even though it said "KVM support yes" on the configure output?)

Not exactly, but close. In the qemu capabilities code, there is a variable called "is_kvm" that is only set if it finds the string "(qemu-kvm-" or "(kvm-" in the help output. It seems to me that this used to be used for a lot more things, but when I look now I see that it's used for very few things (maybe a result of the capabilities code refactoring, or maybe my memory is faulty :-). However, one of the things still in place is that QEMU_CAPS_VNET_HOST isn't set unless is_kvm is true.

I believe this dependency on is_kvm is there because someone said that the feature was only in kvm, but retrospectively that doesn't make sense. Can you try building with the following patch, and see if that works?

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index ea55df5..71a54a5 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -982,7 +982,7 @@ qemuCapsComputeCmdFlags(const char *help,
     if (is_kvm && (version >= 10000 || kvm_version >= 74))
         qemuCapsSet(flags, QEMU_CAPS_VNET_HDR);
-    if (is_kvm && strstr(help, ",vhost=")) {
+    if (strstr(help, ",vhost=")) {
         qemuCapsSet(flags, QEMU_CAPS_VNET_HOST);

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