netdev - Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

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Date:   Mon, 8 Mar 2021 21:20:45 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-rdma@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
        Don Dutile <ddutile@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

On Mon, Mar 08, 2021 at 08:33:03AM -0800, Alexander Duyck wrote:
> On Sun, Mar 7, 2021 at 11:19 AM Leon Romanovsky <leon@...nel.org> wrote:
> >
> > On Sun, Mar 07, 2021 at 10:55:24AM -0800, Alexander Duyck wrote:
> > > On Sun, Feb 28, 2021 at 11:55 PM Leon Romanovsky <leon@...nel.org> wrote:
> > > >
> > > > From: Leon Romanovsky <leonro@...dia.com>
> > > >
> > > > @Alexander Duyck, please update me if I can add your ROB tag again
> > > > to the series, because you liked v6 more.
> > > >
> > > > Thanks
> > > >
> > > > ---------------------------------------------------------------------------------
> > > > Changelog
> > > > v7:
> > > >  * Rebase on top v5.12-rc1
> > > >  * More english fixes
> > > >  * Returned to static sysfs creation model as was implemented in v0/v1.
> > >
> > > Yeah, so I am not a fan of the series. The problem is there is only
> > > one driver that supports this, all VFs are going to expose this sysfs,
> > > and I don't know how likely it is that any others are going to
> > > implement this functionality. I feel like you threw out all the
> > > progress from v2-v6.
> >
> > I'm with you here and tried to present the rationale in v6 when had
> > a discussion with Bjorn, so it is unfair to say "you threw out".
> >
> > Bjorn expressed his preference, and no one came forward to support v6.
>
> Sorry, it wasn't my intention to be accusatory. I'm just not a fan of
> going back to where we were with v1.
>
> With that said, if it is what Bjorn wants then you are probably better
> off going with that. However if that is the direction we are going in
> then you should probably focus on getting his Reviewed-by or Ack since
> he will ultimately be the maintainer for the code.

I hope that he will do it soon.

>
> > >
> > > I really feel like the big issue is that this model is broken as you
> > > have the VFs exposing sysfs interfaces that make use of the PFs to
> > > actually implement. Greg's complaint was the PF pushing sysfs onto the
> > > VFs. My complaint is VFs sysfs files operating on the PF. The trick is
> > > to find a way to address both issues.
> >
> > It is hard to say something meaningful about Greg's complain, he was
> > added in the middle of the discussion without much chances to get full
> > picture.
>
> Right, but what I am getting at is that the underlying problem is that
> you either have sysfs being pushed onto a remote device, or sysfs that
> is having to call into another device. It's not exactly something we
> have had precedent for enabling before, and either perspective seems a
> bit ugly.

I don't agree with the word "ugly", but it is not the point. The point
is that this interface is backed by the ecosystem and must-to-be for the
right SR-IOV utilization.

>
> > >
> > > Maybe the compromise is to reach down into the IOV code and have it
> > > register the sysfs interface at device creation time in something like
> > > pci_iov_sysfs_link if the PF has the functionality present to support
> > > it.
> >
> > IMHO, it adds nothing.
>
> My thought was to reduce clutter. As I mentioned before with this
> patch set we are enabling sysfs for functionality that is currently
> only exposed by one device. I'm not sure it will be used by many
> others or not. Having these sysfs interfaces instantiated at probe
> time or at creation time in the case of VFs was preferable to me.

I said that in v6 to Bjorn, that I expect up to 2-3 vendors to support
this knob. There are not many devices in the market that are comparable
to the mlx5 both in their complexity and adoption.

Thanks