[libvirt] [PATCH 0/6] hostdev: handle usb detach/attach on node

[Daniel Henrique Barboza]

Hi Nikolay,

I was going to review these patches but first I followed up in the
discussions at [1]. Daniel made a few points there:

---
The problems around host USB device passthrough are conceptually similar
to the problems of hots PCI device passthrough.

In both cases we cannot assume the device present on the source device
exists on the target device in the same way.
(...)
For PCI devices we simply refuse to initiate the migration if any host
PCI devices are attached. The mgmt app has to hot-unplug all devices
before migration, and hot-plug new devices after migration if desired.

I'm inclined to suggest that same approach of hotunplug + hotplug either
side of migration is the only viable option for host USB devices too.
(...)
---

My question: is this patch series still alive? Seems to me that you made
these changes based at least partially with migration in mind, and
the migration solution Libvirt adopted is to rely on upper management
to detach/reattach hostdev devices prior/before migration.


Thanks,


DHB


On 8/30/19 9:08 AM, Nikolay Shirokovskiy wrote:
> Hi, all.
>
> This patch series aims to handle case when hostdev usb is detached and then
> attached back on node. Currently libvirtd does not track these events so that in
> the result hostdev usb is not usable.
>
> On the first glace it make sense to delete device from qemu when hostdev is
> detached on node and then add device back when hostdev is attached back. But
> these series adds a different behaviour. On detaching device is deleted from
> qemu and dummy (missing hostaddr and hostbus values) hostdev is inserted, then
> on attaching device on node dummy device is deleted and proper one is inserted.
> This way we have the same semantics of 'missing' flag for hostdev as in case of
> hostdev missing on startup/migration to different host. Also with such
> approach we additionally handle cases when hostdev is missing on startup but
> later is attached to node.
>
> Known issues:
>
> The last patch tries to handle case when hostdev attached/detached when
> libvirtd is down but if fails in one scenario. If hostdev is detached and thus
> dummy device in inserted in qemu then if libvirtd is down and hostdev is
> attached to node then qemu tries to bind appeared hostdev to the dummy device
> but fails as device is missing in qemu's mount namespace. Later when libvirtd
> is up and tries to replace dummy device with actual one the device_add fails
> because libusb the qemu uses tracks that deviced was missed ealier. For this
> reason I proposed to get rid of dummy device in RFC [1]. Alternatively libusb
> can be fixed.
>
> [1] https://www.redhat.com/archives/libvir-list/2019-August/msg01396.html
>
> Nikolay Shirokovskiy (6):
>    qemu: track hostdev delete cause
>    qemu: handle usb hostdev add/del udev events
>    qemu: handle usb hostdev add/del on device DELETED_EVENT
>    conf: parse hostdev missing flag
>    qemu: handle libvirtd restarts during hostdev reattaching
>    qemu: handle hostdev add/del when libvirtd is down
>
>   src/conf/domain_conf.c   |  32 ++++
>   src/conf/domain_conf.h   |  10 ++
>   src/qemu/Makefile.inc.am |   2 +
>   src/qemu/qemu_conf.h     |   3 +
>   src/qemu/qemu_domain.c   |   2 +
>   src/qemu/qemu_domain.h   |   2 +
>   src/qemu/qemu_driver.c   | 376 +++++++++++++++++++++++++++++++++++++--
>   src/qemu/qemu_hotplug.c  |  94 ++++++++--
>   src/qemu/qemu_hotplug.h  |   7 +-
>   src/qemu/qemu_process.c  |  55 +++++-
>   src/util/virhostdev.c    |   6 +-
>   tests/qemuhotplugtest.c  |   2 +-
>   12 files changed, 558 insertions(+), 33 deletions(-)
>