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

[Bodong Wang]

On 3/21/2017 1:01 AM, Alex Williamson wrote:
> On Tue, 21 Mar 2017 16:43:05 +1100
> Gavin Shan <gwshan@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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,
>
> Alex

I agree with Alex on this concern. Sometimes, I need to probe 1 or 2 VFs 
to host side just for development purpose. Bind/unbind satisfy this case 
perfectly, and that's how current implementation is designed. But, from 
Gavin's use case, it will be a pain to bind/unbind hundreds of VFs. So, 
I want to understand how common this use case is. If it's not common, I 
prefer to keep current design because 1) the interface is much easier to 
understand and use 2) less error prone because no need to check current 
total_vfs and maintain a static counter to enable N vfs.

Bodong