Re: [PATCH 12/15] hw/nvme: Initialize capability structures for primary/secondary controllers

["Łukasz Gieryk" <lukasz.gieryk@linux.intel.com>]

On Fri, Nov 05, 2021 at 09:46:28AM +0100, Łukasz Gieryk wrote:
> On Thu, Nov 04, 2021 at 04:48:43PM +0100, Łukasz Gieryk wrote:
> > On Wed, Nov 03, 2021 at 01:07:31PM +0100, Klaus Jensen wrote:
> > > On Oct  7 18:24, Lukasz Maniak wrote:
> > > > From: Łukasz Gieryk <lukasz.gieryk@linux.intel.com>
> > > > 
> > > > With two new properties (sriov_max_vi_per_vf, sriov_max_vq_per_vf) one
> > > > can configure the maximum number of virtual queues and interrupts
> > > > assignable to a single virtual device. The primary and secondary
> > > > controller capability structures are initialized accordingly.
> > > > 
> > > > Since the number of available queues (interrupts) now varies between
> > > > VF/PF, BAR size calculation is also adjusted.
> > > > 
> > > 
> > > While this patch allows configuring the VQFRSM and VIFRSM fields, it
> > > implicitly sets VQFRT and VIFRT (i.e. by setting them to the product of
> > > sriov_max_vi_pervf and max_vfs). Which is just setting it to an upper
> > > bound and this removes a testable case for host software (e.g.
> > > requesting more flexible resources than what is currently available).
> > > 
> > > This patch also requires that these parameters are set if sriov_max_vfs
> > > is. I think we can provide better defaults.
> > > 
> > 
> > Originally I considered more params, but ended up coding the simplest,
> > user-friendly solution, because I did not like the mess with so many
> > parameters, and the flexibility wasn't needed for my use cases. But I do
> > agree: others may need the flexibility. Case (FRT < max_vfs * FRSM) is
> > valid and resembles an actual device.
> > 
> > > How about,
> > > 
> > > 1. if only sriov_max_vfs is set, then all VFs get private resources
> > >    equal to max_ioqpairs. Like before this patch. This limits the number
> > >    of parameters required to get a basic setup going.
> > > 
> > > 2. if sriov_v{q,i}_private is set (I suggested this parameter in patch
> > >    10), the difference between that and max_ioqpairs become flexible
> > >    resources. Also, I'd be just fine with having sriov_v{q,i}_flexible
> > >    instead and just make the difference become private resources.
> > >    Potato/potato.
> > > 
> > >    a. in the absence of sriov_max_v{q,i}_per_vf, set them to the number
> > >       of calculated flexible resources.
> > > 
> > > This probably smells a bit like bikeshedding, but I think this gives
> > > more flexibility and better defaults, which helps with verifying host
> > > software.
> > > 
> > > If we can't agree on this now, I suggest we could go ahead and merge the
> > > base functionality (i.e. private resources only) and ruminate some more
> > > about these parameters.
> > 
> > The problem is that the spec allows VFs to support either only private,
> > or only flexible resources.
> > 
> > At this point I have to admit, that since my use cases for
> > QEMU/Nvme/SRIOV require flexible resources, I haven’t paid much
> > attention to the case with VFs having private resources. So this SR/IOV
> > implementation doesn’t even support such case (max_vX_per_vf != 0).
> > 
> > Let me summarize the possible config space, and how the current
> > parameters (could) map to these (interrupt-related ones omitted):
> > 
> > Flexible resources not supported (not implemented):
> >  - Private resources for PF     = max_ioqpairs
> >  - Private resources per VF     = ?
> >  - (error if flexible resources are configured)
> > 
> > With flexible resources:
> >  - VQPRT, private resources for PF      = max_ioqpairs
> >  - VQFRT, total flexible resources      = max_vq_per_vf * num_vfs
> >  - VQFRSM, maximum assignable per VF    = max_vq_per_vf
> >  - VQGRAN, granularity                  = #define constant
> >  - (error if private resources per VF are configured)
> > 
> > Since I don’t want to misunderstand your suggestion: could you provide a
> > similar map with your parameters, formulas, and explain how to determine
> > if flexible resources are active? I want to be sure we are on the
> > same page.
> > 
> 
> I’ve just re-read through my email and decided that some bits need
> clarification.
> 
> This implementation supports the “Flexible”-resources-only flavor of
> SR/IOV, while the “Private” also could be supported. Some effort is
> required to support both, and I cannot afford that (at least I cannot
> commit today, neither the other Lukasz).
> 
> While I’m ready to rework the Flexible config and prepare it to be
> extended later to handle the Private variant, the 2nd version of these
> patches will still support the Flexible flavor only.
> 
> I will include appropriate TODO/open in the next cover letter.
> 

The summary of my thoughts, so far:
- I'm going to introduce sriov_v{q,i}_flexible and better defaults,
  according to your suggestion (as far as I understand your intentions,
  please correct me if I've missed something).
- The Private SR/IOV flavor, if it's ever implemented, could introduce
  sriov_vq_private_per_vf.
- The updated formulas are listed below.

Flexible resources not supported (not implemented):
 - Private resources for PF     = max_ioqpairs
 - Private resources per VF     = sriov_vq_private_per_vf
 - (error if sriov_vq_flexible is set)

With flexible resources:
 - VQPRT, private resources for PF      = max_ioqpairs - sriov_vq_flexible
 - VQFRT, total flexible resources      = sriov_vq_flexible (if set, or)
                                          VQPRT * num_vfs
 - VQFRSM, maximum assignable per VF    = sriov_max_vq_per_vf (if set, or)
                                          VQPRT
 - VQGRAN, granularity                  = #define constant
 - (error if sriov_vq_private_per_vf is set)

Is this version acceptable?