Re: [PATCH] pci/sriov: Add an option to probe VFs or not before enabling SR-IOV

[Gavin Shan <gwshan@linux.vnet.ibm.com>]

On Tue, Mar 21, 2017 at 12:01:58AM -0600, Alex Williamson wrote:
>On Tue, 21 Mar 2017 16:43:05 +1100
>Gavin Shan <gwshan@linux.vnet.ibm.com> wrote:
>> On Mon, Mar 20, 2017 at 10:57:08PM -0600, Alex Williamson wrote:
>> >On Mon, 20 Mar 2017 18:34:23 -0500
>> >Bodong Wang <bodong@mellanox.com> wrote:  
>> 
>> .../...
>> 
>> >> > Bodong, I'm not sure if there is a requirement to load driver for the
>> >> > specified number of VFs? That indicates no driver will be loaded for
>> >> > other VFs. If so, this interface might serve the purpose as well.    
>> >> Gavin, thanks for the review. That is indeed an interesting suggestion. 
>> >> Theoretically,  we can change that probe_vfs from boolean to integer. 
>> >> And use it as a counter to probe the first N VFs(if N < total_vfs).  
>> >> Let's see if there are any objections.  
>> >
>> >Is it just me or does this seem like a confusing user interface, ie. to
>> >get binary on/off behavior a user now needs to 'cat total_vfs >
>> >sriov_probe_vfs'.  It's not very intuitive, what's the use case for it?
>> >  
>> 
>> After it's changed to integer, it accepts number. If users want to load
>> driver for all VFs and don't want to check the maximal number of VFs,
>> they can simply write 0xffffffff. So "on" and "off" are replaced with 0xffffffff
>> and 0, but users has to press the keyboard more times though.
>> 
>> drivers/net/ethernet/mellanox/mlx4/main.c::probe_vfs_argc allows to specify
>> the number of VFs with which we're going to bind drivers. Less time is needed
>> to enable SRIOV capability. As I had in some development environment: assume
>> PF supports 256 VFs and I'm going to enable all of them, but I only want to
>> load driver for two of them, then test the data path on those two VFs. Besides,
>> I can image the VF needn't a driver in host if it's going to be passed to guest. 
>> Not sure how much sense it makes.
>
>Yes, I understand what you're trying to do, but I still think it's
>confusing for a user interface.  This also doesn't answer what's the
>practical, typical user case you see where it's useful to probe some
>VFs but not others.  The case listed is a development case where you
>could just as easily disable all probing, then manually bind the first
>two VFs to the host driver.  Which is the better design, impose a
>confusing interface on all users to simplify an obscure development
>environment or simplify the user interface and assume developers know
>how to bind devices otherwise?  Thanks,
>

Yeah, your explanation is also fairly reasonable. The interface has
been named as "probe_vfs" instead of "probe_vf" or "probe_vf_driver".
So it seems it should accept number of VFs on which drivers are loaded.
Besides, making this interface accept number corresponds to 3 possiblities:
all, none and load drivers on part of available VFs. So more flexibility
is gained.

User can theoritically have the use case as I had - passing through
some of the VFs to guest: (A) All VFs are bound with drivers; (B) unbind
the drivers for some of the VFs; (C) bind the VFs with vfio-pci; (D) passing
through; (A) is overhead in this scenario. Some CPU cycles are saved if (A)
is avoided.

Thanks,
Gavin