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:   Wed, 24 Feb 2021 09:09:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Leon Romanovsky <leon@...nel.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 Tue, Feb 23, 2021 at 03:07:43PM -0600, Bjorn Helgaas wrote:
> 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.

Hopefully, good luck with that!

> 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.

That seems reasonable, feel free to cc: me on the next patch series and
I'll try to review it, which should make more sense to me than this
email thread :)

thanks,

greg k-h