netdev - Re: [net-next 10/10] net/mlx5e: Add support for PCI relaxed ordering

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Date:   Fri, 26 Jun 2020 15:12:54 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Aya Levin <ayal@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>, linux-pci@...r.kernel.org,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [net-next 10/10] net/mlx5e: Add support for PCI relaxed ordering

On Wed, Jun 24, 2020 at 10:22:58AM -0700, Jakub Kicinski wrote:
> On Wed, 24 Jun 2020 10:34:40 +0300 Aya Levin wrote:
> > >> I think Michal will rightly complain that this does not belong in
> > >> private flags any more. As (/if?) ARM deployments take a foothold
> > >> in DC this will become a common setting for most NICs.  
> > > 
> > > Initially we used pcie_relaxed_ordering_enabled() to
> > >   programmatically enable this on/off on boot but this seems to
> > > introduce some degradation on some Intel CPUs since the Intel Faulty
> > > CPUs list is not up to date. Aya is discussing this with Bjorn.  
> > Adding Bjorn Helgaas
> 
> I see. Simply using pcie_relaxed_ordering_enabled() and blacklisting
> bad CPUs seems far nicer from operational perspective. Perhaps Bjorn
> will chime in. Pushing the validation out to the user is not a great
> solution IMHO.

I'm totally lost, but maybe it doesn't matter because it looks like
David has pulled this series already.

There probably *should* be a PCI core interface to enable RO, but
there isn't one today.

pcie_relaxed_ordering_enabled() doesn't *enable* anything.  All it
does is tell you whether RO is already enabled.

This patch ([net-next 10/10] net/mlx5e: Add support for PCI relaxed
ordering) apparently adds a knob to control RO, but I can't connect
the dots.  It doesn't touch PCI_EXP_DEVCTL_RELAX_EN, and that symbol
doesn't occur anywhere in drivers/net except tg3, myri10ge, and niu.

And this whole series doesn't contain PCI_EXP_DEVCTL_RELAX_EN or
pcie_relaxed_ordering_enabled().

I do have a couple emails from Aya, but they didn't include a patch
and I haven't quite figured out what the question was.

> > > So until we figure this out, will keep this off by default.
> > > 
> > > for the private flags we want to keep them for performance analysis as
> > > we do with all other mlx5 special performance features and flags.