The commit message is longer than it needs to be and the long explanation actually made it harder to comprehend for me. On a Monday in 2023, Laine Stump wrote:
Historically libvirt has treated the concept of "loadable kernel module" and "device driver" as being effectively the same (at least in the case of the vfio-pci driver used for VFIO device assignment). The code assumed that a module named "vfio-pci" implemented a driver named "vfio-pci". In reality, the module is named "vfio_pci", and it implements a device driver named "vfio-pci" (note the difference in separator characters),
The note about the difference in characters is not needed if you mention them explicitly below.
and our code worked only because the modprobe utility we use to load the module will always "normalize" the module name it's given by replacing all "-" (dash) with "_" (underscore) (this has been verified in the modprobe source, which is in the kmod package - there are 3 separate functions that perform this same operation!).
My package manager can tell me which package modprobe is in and I don't need to know the number of functions.
So even though
we asked modprobe to load the "vfio-pci" module, it would actually
load the "vfio_pci" module.
After loading the module with modprobe, libvirt then looks for the
desired *driver* to be present in sysfs, by looking for the directory
/sys/bus/pci/drivers/${driverName}, which would succeed because we
were still looking for the original "dash version" of the name
("vfio-pci").
When we recently gained the ability to manually specify a driver to
bind to with virsh nodedev-detach, the fragility of this system became
apparent - if a user gives the driver name as "vfio_pci", then we
would modprobe the module, but then erroneously believe it hadn't been
loaded because /sys/bus/pci/drivers/vfio_pci didn't exist. For manual
specification of the driver name, this could be dealt with by telling
the user "always use the correct name for the driver, don't assume
that it is the same as the modulename", but it would still end up
confusing people, especially since some drivers do use underscore in
their name (e.g. the mlx5_vfio_pci driver/module).
More more importantly, when we begin looking in the modules.alias file
for the "best" VFIO variant driver for a particular device (in an
upcoming patch), that lookup will provide us with the name of a
*module*, not a driver, and 3 of the 4 examples of vfio-pci/variant
drivers I have access to use underscore in the module name and dash in
the driver, while the 4th uses underscore in both, so 3 out of 4 fail
with the current code.
To make the current code more forgiving (and yet more correct!), as
well as to make the upcoming variant auto-selection actually useful,
this patch follows the following steps:
1) if the requested driver (${driverName}) is present in sysfs drivers
list (/sys/bus/pci/drivers/${driverName}) then use ${driverName} -
DONE
I did not see any response to Jason's points in v1: https://listman.redhat.com/archives/libvir-list/2023-October/242780.html I think we can safely assume that modprobe will accept a dashed name and that module names are using underscores. Is the problem here that you cannot figure out the driver the module provides until after it's loaded? Jano
2) if underscored(${driverName}) (call it ${underName} for short) is
in sysfs drivers list then use ${underName} - DONE
3) if ${underName} *isn't* in the sysfs modules list
(/sys/module/${underName}) then "modprobe ${underName}" to load it.
4) look for the PCI driver implemented by this module, it will
be at /sys/module/${underName}/driver/pci:${newName}.
5) use ${newName} - DONE.
A few notes:
a) This will *always* replace dash with underscore in the name used to
load the module, which seems it could be problematic, but I have
verified this is what modprobe itself does, so I don't think there
will ever be a module with "-" in its name as modprobe would be
unable to load it (the same can't be said for drivers though!)
b) The one case where the above steps wouldn't work would be if
someone implemented a driver within a module where the driver and
module had names that differed other than dash vs. underscore. I
haven't seen an example of this, so the convention seems to be that
they will "match" (modulo - & _). If this mismatch were to ever
occur, though, it could be worked around by simply loading the
module out-of-band during the host system startup, and then using
the driver's name in the config.
c) All of this is conveniently ignoring the possibility of a VFIO
variant driver that is statically linked in the kernel. The entire
design of variant driver auto-detection is based on doing a lookup
in modules.alias, and that only lists *loadable modules* (not
drivers), so unless I'm missing something, it would be impossible
to auto-detect a VFIO variant driver that was statically
linked. This is beyond libvirt's ability to fix; the best that
could be done would be to manually specify the driver name in the
libvirt config, which I suppose is better than nothing :-)
Signed-off-by: Laine Stump <laine@xxxxxxxxxx>
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src/util/virpci.c | 194 +++++++++++++++++++++++++++++++++++++++-------
1 file changed, 164 insertions(+), 30 deletions(-)
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