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Subject: Re: [virtio-dev] Re: [virtio] [PATCH v7 08/11] packed virtqueues: more efficient virtqueue layout
On 01/30/2018 02:50 PM, Cornelia Huck wrote: > On Tue, 23 Jan 2018 02:01:07 +0200 > "Michael S. Tsirkin" <mst@redhat.com> wrote: > >> Performance analysis of this is in my kvm forum 2016 presentation. The >> idea is to have a r/w descriptor in a ring structure, replacing the used >> and available ring, index and descriptor buffer. >> >> This is also easier for devices to implement than the 1.0 layout. >> Several more enhancements will be necessary to actually make this >> efficient for devices to use. >> >> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> >> --- [..] > >> +\subsubsection{Notifying The Device}\label{sec:Basic Facilities >> +of a Virtio Device / Packed Virtqueues / Supplying Buffers to The Device / Notifying The Device} >> + >> +The actual method of device notification is bus-specific, but generally >> +it can be expensive. So the device MAY suppress such notifications if it >> +doesn't need them, using the Driver Event Suppression structure >> +as detailed in section \ref{sec:Basic >> +Facilities of a Virtio Device / Packed Virtqueues / Event >> +Suppression Structure Format}. >> + >> +The driver has to be careful to expose the new \field{flags} >> +value before checking if notifications are suppressed. > > This is all I could find regarding notifications, and it leaves me > puzzled how notifications are actually supposed to work; especially, > where that driver notification structure is supposed to be relayed. > > I'm obviously coming from a ccw perspective, but I don't think that pci > is all that different (well, hopefully). > > Up to now, we notified for a certain virtqueue -- i.e., the device > driver notified the device that there is something to process for a > certain queue. ccw uses the virtqueue number in a gpr for a hypercall, > pci seems to use a write to the config space IIUC. With the packed > layout, we have more payload per notification. We should be able to put > it in the same gpr than the virtqueue for ccw (if needed, with some > compat magic, or with a new hypercall, which would be ugly but doable). > Not sure how this is supposed to work with pci. > > Has there been any prototyping done to implement this in qemu + KVM? > I'm unsure how this will work with ioeventfds, which just trigger. > I'm also interested in an answer to Connie's question regarding a QEMU + KVM prototype. IMHO we should definitively have at least a such an prototype (preferably a reasonable implementation) before voting about the changes envisioned by this series. [META] Unfortunately I have skipped v6 altogether (that is not even lurker mode). I'm a bit overwhelmed. I'm also in doubt about how to articulate my feelings and opinions. Maybe I will wait for v8 with my comments. You seem to have received enough comments for v7 already. Anyway, I'm happy to see virtio version 1.1 is slowly materializing. Regards, Halil
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