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

[Laine Stump]

On 08/15/2011 09:21 AM, Daniel Veillard wrote:
> On Sun, Aug 14, 2011 at 03:43:52AM -0400, Laine Stump wrote:
>> 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).
>    This smells like an API available just as a safety belt for code
> not fully tested in the field. So I have the same dislike for
> consolidating this as a full long term libvirt API. But that's not
> the first one, and I'm afraid it's not the last one, and it affects
> only the XML domain with an extra attribute so the damage is limited.
>
> [...]
>> +<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>
>    Okay at least it's clear in the documentation
>
>    Code looks correct, there is one example trying to test both values
> and for the 2 use cases, then fine,
>
>    ACK.

Thanks, I pushed it.