Re: [libvirt] [PATCH] qemu: support event_idx parameter for virtio disk and net devices

In some versions of qemu, both virtio-blk-pci and virtio-net-pci devices can have an event_idx setting that determines some details of event processing. When it is enabled, it "reduces the number of interrupts and exits for the guest". qemu will automatically enable this feature when it is available, but there may be cases where this new feature could actually make performance worse (NB: no such case has been found so far). As a safety switch in case such a situation is encountered in the field, this patch adds a new attribute "event_idx" to the <driver> element of both disk and interface devices. event_idx can be set to "on" (to force event_idx on in case qemu has it disabled by default) or "off" (for force event_idx off). In the case that event_idx support isn't present in qemu, the attribute is ignored (this on the advice of the qemu developer).

+ <li> + The optional <code>event_idx</code> attribute controls + some aspects of device event processing. The value can be + either 'on' or 'off' - if it is on, it will reduce the + number of interupts and exits for the guest. The default + is determined by QEMU; usually if the feature is + supported, default is on. In case there is a situation + where this behavior is suboptimal, this attribute provides + a way to force the feature off. + <span class="since">Since 0.9.5 (QEMU and KVM only)</span> + <b>In general you should leave this option alone, unless you + are very certain you know what you are doing.</b> + </li>