4:48 p.m.
On Mon, 22 Feb 2021 11:08:12 +0100
Erik Skultety <eskultet(a)redhat.com> wrote:
On Wed, Feb 03, 2021 at 11:38:44AM -0600, Jonathon Jongsma wrote:
> This patch series follows the previously-merged series which added
> support for transient mediated devices. This series expands mdev
> support to include persistent device definitions. Again, it relies
> on mdevctl as the backend.
>
> It follows the common libvirt pattern of APIs by adding the
> following new APIs for node devices:
> - virNodeDeviceDefineXML() - defines a persistent device
> - virNodeDeviceUndefine() - undefines a persistent device
> - virNodeDeviceCreate() - starts a previously-defined device
So, I tested the patches on Friday and have a few notes:
- all of the driver implementations mentioned ^above need to pass an
error buffer to the respective mdevctl wrapper, because the generic
error "Unable to XZY mediated device" doesn't help at all. Kind of
like with QEMU errors, we need to extract it from an error buffer
(you already have input and output, so just add another one) --> this
relates to patches 14,18,21
- trying to undefine an active mdev reports an error in libvirt
--> this is both inconsistent with the rest of libvirt's
subsystems and there's also no reason for it since mdevctl supports it
--> we need to support transient devices as well
- trying to define the following XML hangs for 5s and then fails:
<device>
<parent>pci_0000_06_00_0</parent>
<driver>
<name>vfio_mdev</name>
</driver>
<capability type='mdev'>
<type id='nvidia-11'/>
<uuid>d41787d2-2e0e-4021-8ed2-b6f1ef305a9f</uuid>
</capability>
</device>
I assume that you have a parent device that supports creating an mdev
of this type? In other words, this was expected to succeed, right?
-> I debugged this on Friday evening and for some reason
driver->devs hash table of devices was NULL going through the
nodeDeviceFindNewMediatedDevice call, but I haven't managed to figure
out why it was NULL, listing devices worked just fine
So, I often get a 5s delay when trying to define a new mdev. But it
always succeeds after the delay.
The reason for the delay is that the device is generally not in our
device list the first time we call nodeDeviceFindNewMediatedDevice().
It's usually not in our device list because the device is only added
to the driver's device list after we get a notification from mdevctl
(via monitoring the mdevctl config directory) and then we re-query
mdevctl for the list of defined devices. Because mdevctl is not
blazingly fast[1], and because we do the querying in a separate thread,
the new device has usually not been discovered when we first call
nodeDeviceFindNewMediatedDevice(). So we sleep for 5s before trying
again.
For udev devices, this 5s delay is usually not hit because
nodeDeviceFindNewDevice() first calls 'udevadm settle' before
checking the device list. This waits just long enough to ensure that
pending udev events are handled. Unfortunately we don't have a similar
"wait just long enough" command for mdevctl, so we almost always hit
the 5s retry timeout.
There are a couple of potential ways to avoid hitting this 5s timeout:
1. directly add a placeholder device to the driver's device list
immediately after calling 'mdevctl define' so that it is guaranteed to
be in the device list when we call nodeDeviceFindNewMediatedDevice().
We can then update that device definition with additional info
when mdevctl is eventually queried.
2. don't wait 5s every time a device is not found. Instead, start with
a small sleep timeout and increase it gradually until we hit the max
timeout. In other words, instead of check; sleep(5); check; sleep(5);
check; sleep(5); we could instead do something like: check; sleep(1);
check; sleep(2); check; sleep(4); check; sleep(8)...
[1]$ time mdevctl list --defined
49bf2346-6747-4ad6-be5a-adc2f0a10b5c 0000:00:02.0 i915-GVTg_V5_8 manual
4fcd3666-e58a-4c63-969c-bd616a158c0d 0000:00:02.0 i915-GVTg_V5_8 manual
5c152919-3a34-4338-b7c9-532f73fa5440 0000:00:02.0 i915-GVTg_V5_4 manual
real 0m0.782s
user 0m0.735s
sys 0m0.056s
- I also prepared several adjustments to how we define the mdevctl
wrappers + some test adjustments which I wanted to share with you,
but I haven't tested those thoroughly since I was debugging why that
XML couldn't be defined, I'll share it when we eliminate the
underlying problem -> if you're wondering why I didn't just put it
into the review, well, I figured sharing the code was more
descriptive than if I used words
Would you like me to wait for this before sending a new series?
Thanks,
Jonathon