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Date:   Sat, 11 Jul 2020 21:58:38 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Rajat Jain <rajatxjain@gmail.com>
Cc:     Rajat Jain <rajatja@google.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
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        Bjorn Helgaas <bhelgaas@google.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
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        Heikki Krogerus <heikki.krogerus@linux.intel.com>
Subject: Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for
 untrusted/external-facing devices
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On Sat, Jul 11, 2020 at 05:08:51PM -0700, Rajat Jain wrote:
> On Sat, Jul 11, 2020 at 12:53 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Fri, Jul 10, 2020 at 03:53:59PM -0700, Rajat Jain wrote:
> > > On Fri, Jul 10, 2020 at 2:29 PM Raj, Ashok <ashok.raj@intel.com> wrote:
> > > > On Fri, Jul 10, 2020 at 03:29:22PM -0500, Bjorn Helgaas wrote:
> > > > > On Tue, Jul 07, 2020 at 03:46:04PM -0700, Rajat Jain wrote:
> > > > > > When enabling ACS, enable translation blocking for external facing ports
> > > > > > and untrusted devices.
> > > > > >
> > > > > > Signed-off-by: Rajat Jain <rajatja@google.com>
> > > > > > ---
> > > > > > v4: Add braces to avoid warning from kernel robot
> > > > > >     print warning for only external-facing devices.
> > > > > > v3: print warning if ACS_TB not supported on external-facing/untrusted ports.
> > > > > >     Minor code comments fixes.
> > > > > > v2: Commit log change
> > > > > >
> > > > > >  drivers/pci/pci.c    |  8 ++++++++
> > > > > >  drivers/pci/quirks.c | 15 +++++++++++++++
> > > > > >  2 files changed, 23 insertions(+)
> > > > > >
> > > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > > > index 73a8627822140..a5a6bea7af7ce 100644
> > > > > > --- a/drivers/pci/pci.c
> > > > > > +++ b/drivers/pci/pci.c
> > > > > > @@ -876,6 +876,14 @@ static void pci_std_enable_acs(struct pci_dev *dev)
> > > > > >     /* Upstream Forwarding */
> > > > > >     ctrl |= (cap & PCI_ACS_UF);
> > > > > >
> > > > > > +   /* Enable Translation Blocking for external devices */
> > > > > > +   if (dev->external_facing || dev->untrusted) {
> > > > > > +           if (cap & PCI_ACS_TB)
> > > > > > +                   ctrl |= PCI_ACS_TB;
> > > > > > +           else if (dev->external_facing)
> > > > > > +                   pci_warn(dev, "ACS: No Translation Blocking on external-facing dev\n");
> > > > > > +   }
> > > > >
> > > > > IIUC, this means that external devices can *never* use ATS and
> > > > > can never cache translations.
> > >
> > > Yes, but it already exists today (and this patch doesn't change that):
> > > 521376741b2c2 "PCI/ATS: Only enable ATS for trusted devices"
> > >
> > > IMHO any external device trying to send ATS traffic despite having ATS
> > > disabled should count as a bad intent. And this patch is trying to
> > > plug that loophole, by blocking the AT traffic from devices that we do
> > > not expect to see AT from anyway.
> >
> > Thinking about this some more, I wonder if Linux should:
> >
> >   - Explicitly disable ATS for every device at enumeration-time, e.g.,
> >     in pci_init_capabilities(),
> >
> >   - Enable PCI_ACS_TB for every device (not just external-facing or
> >     untrusted ones),
> >
> >   - Disable PCI_ACS_TB for the relevant devices along the path only
> >     when enabling ATS.
> >
> > One nice thing about doing that is that the "untrusted" test would be
> > only in pci_enable_ats(), and we wouldn't need one in
> > pci_std_enable_acs().
> 
> Yes, this could work.
> 
> I think I had thought about this but I'm blanking out on why I had
> given it up. I think it was because of the possibility that some
> bridges may have "Translation blocking" disabled, even if not all
> their descendents were trusted enough to enable ATS on them. But now
> thinking about this again, as long as we retain the policy of not
> enabling ATS on external devices (and thus enable TB for sure on
> them), this should not be a problem. WDYT?

I think I would feel better if we always enabled Translation Blocking
except when we actually need it for ATS.  But I'm not confident about
how all the pieces of ATS work, so I could be missing something.

> > It's possible BIOS gives us devices with ATS enabled, and this
> > might break them, but that seems like something we'd want to find
> > out about.
> 
> Why would they break? We'd disable ATS on each device as we
> enumerate them, so they'd be functional, just with ATS disabled
> until it is enabled again on internal devices as needed. Which would
> be WAI behavior?

If BIOS handed off with ATS enabled and we somehow relied on it being
already enabled, something might break if we start disabling ATS.
Just a theoretical possibility, doesn't seem likely to me.

Bjorn