netdev - Re: [PATCH mlx5-next v6 1/4] PCI: Add sysfs callback to allow MSI-X table size change of SR-IOV VFs

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Date:   Tue, 23 Feb 2021 15:07:43 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, linux-pci@...r.kernel.org,
        linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        Don Dutile <ddutile@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Krzysztof Wilczyński <kw@...ux.com>
Subject: Re: [PATCH mlx5-next v6 1/4] PCI: Add sysfs callback to allow MSI-X
 table size change of SR-IOV VFs

On Sun, Feb 21, 2021 at 08:59:18AM +0200, Leon Romanovsky wrote:
> On Sat, Feb 20, 2021 at 01:06:00PM -0600, Bjorn Helgaas wrote:
> > On Fri, Feb 19, 2021 at 09:20:18AM +0100, Greg Kroah-Hartman wrote:
> >
> > > Ok, can you step back and try to explain what problem you are trying to
> > > solve first, before getting bogged down in odd details?  I find it
> > > highly unlikely that this is something "unique", but I could be wrong as
> > > I do not understand what you are wanting to do here at all.
> >
> > We want to add two new sysfs files:
> >
> >   sriov_vf_total_msix, for PF devices
> >   sriov_vf_msix_count, for VF devices associated with the PF
> >
> > AFAICT it is *acceptable* if they are both present always.  But it
> > would be *ideal* if they were only present when a driver that
> > implements the ->sriov_get_vf_total_msix() callback is bound to the
> > PF.
> 
> BTW, we already have all possible combinations: static, static with
> folder, with and without "sriov_" prefix, dynamic with and without
> folders on VFs.
> 
> I need to know on which version I'll get Acked-by and that version I
> will resubmit.

I propose that you make static attributes for both files, so
"sriov_vf_total_msix" is visible for *every* PF in the system and
"sriov_vf_msix_count" is visible for *every* VF in the system.

The PF "sriov_vf_total_msix" show function can return zero if there's
no PF driver or it doesn't support ->sriov_get_vf_total_msix().
(Incidentally, I think the documentation should mention that when it
*is* supported, the contents of this file are *constant*, i.e., it
does not decrease as vectors are assigned to VFs.)

The "sriov_vf_msix_count" set function can ignore writes if there's no
PF driver or it doesn't support ->sriov_get_vf_total_msix(), or if a
VF driver is bound.

Any userspace software must be able to deal with those scenarios
anyway, so I don't think the mere presence or absence of the files is
a meaningful signal to that software.

If we figure out a way to make the files visible only when the
appropriate driver is bound, that might be nice and could always be
done later.  But I don't think it's essential.

Bjorn