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

From: Łukasz Gieryk
Subject: Re: [PATCH 12/15] hw/nvme: Initialize capability structures for primary/secondary controllers
Date: Thu, 4 Nov 2021 16:48:43 +0100
User-agent: Mutt/1.9.4 (2018-02-28)

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.


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