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

From: Bjorn Helgaas <helgaas@kernel.org>
To: Leon Romanovsky <leon@kernel.org>
Cc: "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Saeed Mahameed" <saeedm@nvidia.com>,
	"Jason Gunthorpe" <jgg@nvidia.com>,
	"Alexander Duyck" <alexander.duyck@gmail.com>,
	"Jakub Kicinski" <kuba@kernel.org>,
	linux-pci@vger.kernel.org, linux-rdma@vger.kernel.org,
	netdev@vger.kernel.org, "Don Dutile" <ddutile@redhat.com>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"David S . Miller" <davem@davemloft.net>,
	"Krzysztof Wilczyński" <kw@linux.com>
Subject: Re: [PATCH mlx5-next v6 1/4] PCI: Add sysfs callback to allow MSI-X table size change of SR-IOV VFs
Date: Tue, 23 Feb 2021 15:07:43 -0600	[thread overview]
Message-ID: <20210223210743.GA1475710@bjorn-Precision-5520> (raw)
In-Reply-To: <YDIExpismOnU3c4k@unreal>

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