On Thu, May 19, 2011 at 09:44:35AM +0100, Stefan Hajnoczi wrote:
On Thu, May 19, 2011 at 8:26 AM, Daniel Veillard <veillard@redhat.com> wrote:
On Wed, May 18, 2011 at 04:07:30PM +0800, Daniel Veillard wrote:
On Tue, May 17, 2011 at 03:56:11PM +0100, Daniel P. Berrange wrote:
On Tue, May 17, 2011 at 04:49:17PM +0200, Michal Privoznik wrote:
This feature allows QEMU to achieve higher throughput, but is available only in recent versions. It is accessible via ioeventfd attribute with accepting values 'on', 'off'. Only experienced users needs to set this, because QEMU defaults to 'on', meaning higher performance. Translates into virtio-{blk|net}-pci.ioeventfd option. [...] + <li> + The optional <code>ioeventfd</code> attribute enables or disables + IOEventFD feature for virtqueue notify. The value can be either + 'on' or 'off'. + <span class="since">Since 0.9.2 (QEMU and KVM only)</span>
This is a qemu specific attribute name & description. IMHO we shouldn't be exposing that directly. Who even knows what effect it actually has on the guests...
Agreed, what is the semantic of this flag, beside allowing to switch something in qemu ?
Just to clarify my answer a bit, the problem here is that the patch does not explain what the ioeventfd qemu flag does in practice and how it influence the virtualization. To be able to provide a good API and maintain it long term we need to be able to explain the semantic of the API (be it a function of the library or part of the XML being used), only then we can guarantee that there is no misunderstanding about what it does, and also allow us to reuse it in case the same functionality is provided by another hypervisor. So instead of explaining the option using terms from QEmu, let's explain what it does in general terms and use those general terms to model the API,
I don't think there is a general API here, ioeventfd is specific to QEMU's architecture. It allows you to switch between two internal threading models for handling I/O emulation. It could change in the future if QEMU's architecture changes. This is not an end-user feature, it's more an internal performance tunable.
Actually reading about it at https://patchwork.kernel.org/patch/43390/ it seems that can be described as "domain I/O asynchronous handling", it's a shortcut because it's not for the whole I/O only a part of it but that in itself is sufficiently generic to be potentially useful for something else. I would just suggest to rename the attribute "asyncio" with value on or off, document the fact that it allows to force on or off some of the asynchronous I/O handling for the device, and that the default is left to the discretion of the hypervisor. In case we need to refine later, we can still provide a larger set of accepted values for that attribute, assuming people really want to make more distinctive tuning, Daniel -- Daniel Veillard | libxml Gnome XML XSLT toolkit http://xmlsoft.org/ daniel@veillard.com | Rpmfind RPM search engine http://rpmfind.net/ http://veillard.com/ | virtualization library http://libvirt.org/