|
On 7/18/19 11:56 AM, Daniel Henrique
Barboza wrote:
On 7/18/19 12:29 PM, Laine Stump
wrote:
On 7/18/19 10:29 AM, Daniel
Henrique Barboza wrote:
Hi,
I have a PoC that enables partial coldplug assignment of
multifunction
PCI devices with managed mode. At this moment, Libvirt can't
handle
this scenario - the code will detach only the hostdevs from
the XML,
when in fact the whole IOMMU needs to be detached. This can
be
verified by the fact that Libvirt handles the unmanaged
scenario
well, as long as the user detaches the whole IOMMU
beforehand.
I have played with 2 approaches. The one I am planning to
contribute
back is a change inside virHostdevGetPCIHostDeviceList(),
that
adds the extra PCI devices for detach/re-attach in case a
PCI
Multifunction device in managed mode is presented in the
XML.
If you're thinking of doing that
automatically, then I should warn you that we had discussed
that a long time ago, and decided that it was a bad idea to
do it because it was likely someone would, e.g. try to
assign an audio device to their guest that happened to be
one function on a multifunction device that also contained a
disk controller (or some other device) that the host needed
for proper operation.
Let's say that I have a Multi PCI card with 4 functions, and I
want a guest to use
only the function 0 of that card. At this moment, I'm only able to
do that if I
manually execute nodedev-detach on all 4 functions beforehand and
use function
0 as a hostdev with managed=false.
What I've implemented is a way of doing the detach/re-attach of
the whole IOMMU
for the user, if the hostdev is set with managed=true (and perhaps
I should also
consider verifying the 'multifunction=yes' attribute as well, for
more clarity).
I am not trying to assign all the IOMMU devices to the guest - not
sure if that's
what you were talking about up there, but I'm happy to emphasize
that's not
the case.
No, we're talking about the same thing. We specifically talked
about the possibility of doing exactly this several years ago, and
decided against it.
Now, yes, if the user is unaware of the consequences of detaching
all devices
of the IOMMU from the host, bad things can happen. If that's what
you're saying,
fair enough. I can make an argument about how we can't shield the
user from
his/her own 'unawareness' forever, but in the end it's better to
be on the safe
side.
We really shouldn't do anything with any host device if it's not
explicitly given in the config.
It may be that in *your* particular case, you
understand that the functions you don't want to assign to
the guest are not otherwise used, and it's not dangerous to
suddenly detach them from their host driver. But you can't
assume that will always be the case.
If you *really* can't accept just assigning
all the devices in that IOMMU group to the guest (thus
making them all explicitly listed in the config, and obvious
to the administrator that they won't be available on the
host) and simply not using them, then you either need to
separately detach those particular functions from the host,
or come up with a way of having the domain config explicitly
list them as "detached from the host but not actually
attached to the guest".
I can live with that - it will automate the detach/re-attach
process, which is
my goal here, and it force the user to know exactly what is going
to be detached
from the host, minimizing errors. If no one is against adding an
extra
parameter 'unassigned=true' to the hostdev in these cases, I can
make this
happen.
I don't have any idealogical opinion against that (maybe there's
a better name for the attribute, but I can't think of it).
But to back up a bit - what is it about managed='yes' that makes
you want to do it that way instead of managed='no'? Do you really
ever need the devices to be binded to the host driver? Or are you
just using managed='yes' because there's not a standard/concenient
place to configure devices to be permanently binded to vfio-pci
immediately when the host boots? Truthfully, a great majority of
the most troubling bugs with device assignment are due to use of
managed='yes', since it exercises the kernel's device driver
binding/unbinding code so much, and reveals strange races in the
(usually kernel) code, but in almost all cases the devices being
assigned to guests are *never* used directly by the host anyway,
so there is no point in repeatedly rebinding the host driver to
the device - it just sits there unused [1] until the next time it
is needed by a guest, and at that time it gets rebinded to
vfio-pci, rinse, repeat.
I think we should spend more time making it easier to have
devices "pre-binded" to vfio-pci at boot time, so that we could
discourage use of managed='yes'. (not "instead of" what you're
doing, but "in addition to" it).
[1] (in the case of network device VFs, often it isn't "unused",
but instead is *improperly* used on the host due to NetworkManager
insisting on setting the device IFF_UP and starting up a DHCP
client. So it's not just finding races in the kernel driver
binding/initialization code, but also falling prey to (imho) the
poor choice of NM to force all interfaces up and default to
running dhcp on all unconfigured interfaces)
|
