Re: [dpdk-dev, RFC] drivers: advertise kmod dependencies in pmdinfo

[Olivier Matz <olivier.matz@6wind.com>]

Hi Neil,

On 08/31/2016 03:27 PM, Neil Horman wrote:
> On Wed, Aug 31, 2016 at 11:21:18AM +0200, Olivier Matz wrote:
>> Hi Neil,
>>
>> On 08/30/2016 03:23 PM, Neil Horman wrote:
>>> On Fri, Aug 26, 2016 at 03:20:46PM +0200, Olivier Matz wrote:
>>>> Add a new macro DRIVER_REGISTER_KMOD_DEP() that allows a driver to
>>>> declare the list of kernel modules required to run properly.
>>>>
>>>> Today, most PCI drivers require uio/vfio.
>>>>
>>>> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
>>>>
>>>> ---
>>>> In this RFC, I supposed that all PCI drivers require a the loading of a
>>>> uio/vfio module (except mlx*), this may be wrong.
>>>> Comments are welcome!
>>>>
>>>>
>>>>  buildtools/pmdinfogen/pmdinfogen.c      |  1 +
>>>>  buildtools/pmdinfogen/pmdinfogen.h      |  1 +
>>>>  drivers/crypto/qat/rte_qat_cryptodev.c  |  2 ++
>>>>  drivers/net/bnx2x/bnx2x_ethdev.c        |  4 ++++
>>>>  drivers/net/bnxt/bnxt_ethdev.c          |  2 ++
>>>>  drivers/net/cxgbe/cxgbe_ethdev.c        |  2 ++
>>>>  drivers/net/e1000/em_ethdev.c           |  2 ++
>>>>  drivers/net/e1000/igb_ethdev.c          |  4 ++++
>>>>  drivers/net/ena/ena_ethdev.c            |  2 ++
>>>>  drivers/net/enic/enic_ethdev.c          |  2 ++
>>>>  drivers/net/fm10k/fm10k_ethdev.c        |  2 ++
>>>>  drivers/net/i40e/i40e_ethdev.c          |  2 ++
>>>>  drivers/net/i40e/i40e_ethdev_vf.c       |  2 ++
>>>>  drivers/net/ixgbe/ixgbe_ethdev.c        |  4 ++++
>>>>  drivers/net/mlx4/mlx4.c                 |  2 ++
>>>>  drivers/net/mlx5/mlx5.c                 |  3 +++
>>>>  drivers/net/nfp/nfp_net.c               |  2 ++
>>>>  drivers/net/qede/qede_ethdev.c          |  4 ++++
>>>>  drivers/net/szedata2/rte_eth_szedata2.c |  2 ++
>>>>  drivers/net/thunderx/nicvf_ethdev.c     |  2 ++
>>>>  drivers/net/virtio/virtio_ethdev.c      |  2 ++
>>>>  drivers/net/vmxnet3/vmxnet3_ethdev.c    |  2 ++
>>>>  lib/librte_eal/common/include/rte_dev.h | 14 ++++++++++++++
>>>>  tools/dpdk-pmdinfo.py                   |  5 ++++-
>>>>  24 files changed, 69 insertions(+), 1 deletion(-)
>>>>
>>>
>>> Generally speaking, I like the idea, it makes sense to me in terms of using
>>> pmdinfo to export this information
>>>
>>> That said, This may need to be a set of macros.  By that I mean (and correct me
>>> if I'm wrong here), but the relationship between pmd's and kernel modules is in
>>> some cases, more complex than a 'requires' or 'depends' relationship.  That is
>>> to say, some pmd may need user space hardware access, but can use either uio OR
>>> vfio, but doesn't need both, and can continue to function if only one is
>>> available.  Other PMD's may be able to use vfio or uio, but can still function
>>> without either.  And some, as your patch implements, simply require one or the
>>> other to function.  As such it seems like you may want a few macros, in the form
>>> of:
>>>
>>> DRIVER_REGISTER_KMOD_REQUEST - List of modules to attempt loading, ignore any
>>> failures 
>>> DRIVER_REGISTER_KMOD_REQUIRE - List of modules required to be loaded after
>>> request macro completes, fail if any are not loaded
>>>
>>> Thats just spitballing, mind you, theres probably a better way to do it, but the
>>> idea is to list a set of modules you would like to have, and then create a
>>> parsable syntax to describe the modules that need to be loaded after the request
>>> is complete so that you can accurately codify the situations I described above.
>>
>> Thank you for your feedback.
>> However, I'm not sure I'm perfectly getting what you suggest.
>>
>> Do you think some PMDs could request a kernel module without really
>> requiring it? Do you have an example in mind?
>>
> Yes, thats precisely it.  The most clear example I could think of (though I'm
> not sure if any pmd currently supports this), is a pmd that supports both UIO
> and VFIO communication with the kernel.  Such a PMD requires that one of those
> two modules be loaded, but only one (i.e. both are not required), so if only the
> uio kernel module loads is a success case, likewise if only the vfio module
> loads can be treated as success.  Both loading are clearly successful.  Only if
> neither load do we have a failure case.  I'm suggesting that the grammer that
> your exports define should take those cases into account.  Its not always as
> simple as "I must have the following modules"
> 
>> The syntax I've submitted lets you define several lists of modules, so
>> that the user or the script that starts the application can decide which
>> kmod list is better according to the environment.
>>
> If you have a human intervening in the module load process, sure, then its fine.
> But it seems that this particular feature that you're implemnting might have
> automated uses.  That is to say the dpdk core library might be interested in
> parsing this particular information to direct module autoloading, and if thats
> desireable then you need to define these lists such that you can codify failure
> and success conditions.
> 
>> For example, most drivers will advertise
>> "uio,igb_uio:uio,uio_pci_generic:vfio,vfio-pci", and the user or script
>> will have to choose between loading:
>> - uio igb_uio
>> - uio uio_pci_generic
>> - vfio vfio-pci
>>
> Oh, I see, so your list is a colon delimited list of module load sets, where at
> least one set must succeed by loading all modules in its set, but the failure of
> any one set isn't fatal to the process?  e.g. a string like this:
> 
> uio,igb_uio:vfio,vfio-pci
> 
> could be interpreted to mean "I must load (uio AND igb_uio) OR (vfio AND
> vfio-pci).  If the evaluation of that statement results in false, then the
> operation fails, otherwise it succedes.
> 
> If thats the case, then, apologies, we're on the same page, and this will work
> just fine.

Yep, that's the idea.

Colon and commas are the best separators I've thought about, but any
idea to make the syntax clearer is welcome ;)

Maybe a syntax like is clearer:
  "(mod1 & mod2)|(mod3 & mod4)" ?
But it would let the user think that more complex expressions are valid,
like "(mod1 & (mod2 | mod3)) | mod4", which is probably overkill.

Regards,
Olivier