Date:   Mon, 31 Jul 2017 15:55:07 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Feng Kan <fkan@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH V3] pci: quirk: Apply APM ACS quirk to XGene devices

On Mon, 31 Jul 2017 10:56:53 -0700
Feng Kan <fkan@....com> wrote:

> On Fri, Jul 28, 2017 at 4:00 PM, Alex Williamson
> <alex.williamson@...hat.com> wrote:
> > On Fri, 28 Jul 2017 11:50:43 -0700
> > Feng Kan <fkan@....com> wrote:
> >  
> >> The APM X-Gene PCIe root port does not support ACS at this point.
> >> However, the hw provides isolation and source validation through
> >> the SMMU. The stream ID generated by the PCIe ports contain both
> >> the BDF as well as the port ID in its 3 most significant bits.
> >> Turn on ACS but disable all the peer to peer features.
> >>
> >> Signed-off-by: Feng Kan <fkan@....com>
> >> ---
> >>       V3 Change: Add comment regarding unique port id in stream ID
> >>       V2 Change: Move XGene ACS quirk to unique XGene function
> >>
> >>  drivers/pci/quirks.c | 15 +++++++++++++++
> >>  1 file changed, 15 insertions(+)
> >>
> >> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> >> index 085fb78..0f8f1cd 100644
> >> --- a/drivers/pci/quirks.c
> >> +++ b/drivers/pci/quirks.c
> >> @@ -4120,6 +4120,19 @@ static int pci_quirk_cavium_acs(struct pci_dev *dev, u16 acs_flags)
> >>       return acs_flags ? 0 : 1;
> >>  }
> >>
> >> +static int pci_quirk_xgene_acs(struct pci_dev *dev, u16 acs_flags)
> >> +{
> >> +     /*
> >> +      * XGene root matching this quirk do not allow peer-to-peer
> >> +      * transactions with others, allowing masking out these bits as if they
> >> +      * were unimplemented in the ACS capability.
> >> +      */
> >> +     acs_flags &= ~(PCI_ACS_SV | PCI_ACS_TB | PCI_ACS_RR |
> >> +                    PCI_ACS_CR | PCI_ACS_UF | PCI_ACS_DT);
> >> +
> >> +     return acs_flags ? 0 : 1;
> >> +}
> >> +
> >>  /*
> >>   * Many Intel PCH root ports do provide ACS-like features to disable peer
> >>   * transactions and validate bus numbers in requests, but do not provide an
> >> @@ -4368,6 +4381,8 @@ static int pci_quirk_mf_endpoint_acs(struct pci_dev *dev, u16 acs_flags)
> >>       { 0x10df, 0x720, pci_quirk_mf_endpoint_acs }, /* Emulex Skyhawk-R */
> >>       /* Cavium ThunderX */
> >>       { PCI_VENDOR_ID_CAVIUM, PCI_ANY_ID, pci_quirk_cavium_acs },
> >> +     /* APM XGene */
> >> +     { PCI_VENDOR_ID_AMCC, 0xE004, pci_quirk_xgene_acs },
> >>       { 0 }
> >>  };
> >>  
> >
> > Hi Feng,
> >
> > Sorry, I have one more question as I happened to spend some time
> > looking at PCI_ACS_DT this week.  DT specifies that peer-to-peer should
> > occur normally between egress ports for transactions which are
> > pre-translated by an ATS unit on the endpoint.  Therefore if a root
> > port doesn't allow peer-to-peer, it seems like it should not claim to
> > support PCI_ACS_DT.  I know your quirk is just a copy of the Cavium
> > one, but we should also go back and verify this question with them, or
> > perhaps I'm misinterpreting this capability.  AIUI this is also a
> > performance capability, not an isolation capability, so it shouldn't
> > affect the ability to consider a device isolated, it only gets
> > confusing if we expect a performance benefit from this but don't
> > actually see one.  Does your root port have this ability to
> > selectively allow peer-to-peer of pre-translated transactions?  Thanks,  
> We do not support peer to peer between root ports (each root port is a
> root complex in itself).
> Therefore, this bit set/unset has no effect.
> 
> As one of our guys pointed out in PCIe 3.1a, it may help address your
> concern above.
> 
> """
> Root Port indication of ACS Direct Translated P2P support does not imply any
> particular level of peer-to-peer support by the Root Complex, or that
> peer-to-peer traffic is supported at all.
> """

That's interesting, but is that referring to the ACS capability or the
control?  I could imagine how advertising the capability would not
imply any particular level of p2p, but enabling the control (which is
what we're claiming via this quirk) the spec states:

  When ACS Direct Translated P2P is enabled in a Downstream Port,
  peer-to-peer Memory Requests whose Address Type (AT) field indicates
  a Translated address must be routed normally (“directly”) to the peer
  Egress Port, regardless of ACS P2P Request Redirect and ACS P2P
  Egress Control settings. All other peer-to-peer Requests must still
  be subject to ACS P2P Request Redirect and ACS P2P Egress Control
  settings.

Note the *must* phrasing.  Furthermore, 7.16.3:

  ACS Direct Translated P2P Enable (T) – When Set, overrides the ACS
  P2P Request Redirect and ACS P2P Egress Control mechanisms with
  peer-to-peer Memory Requests whose Address Translation (AT) field
  indicates a Translated address (see Section 6.12.3).

  This bit is ignored if ACS Translation Blocking Enable (B) is 1b.

  Default value of this bit is 0b. Must be hardwired to 0b if the ACS
  Direct Translated P2P functionality is not implemented.

Note the 2nd sentence, DT is ignored if TB is 1, which you're also
claiming is enabled with the mask in the patch above.  So the
functionality is not implemented in hardware, exposing the control as
enabled seems to violate a must condition in the spec, and all is for
naught because the bit combination causes it to be ignored anyway.

That leads to the question of why the patch above advertises
PCI_ACS_TB, where the same section as above describes:

  ACS Translation Blocking Enable (B) – When Set, the component blocks
  all Upstream Memory Requests whose Address Translation (AT) field is
  not set to the default value.

  Default value of this bit is 0b. Must be hardwired to 0b if the ACS
  Translation Blocking functionality is not implemented.

Does the root port block transactions with the AT field set to a
non-default value?  It seems that this would effectively nullify ATS on
a downstream endpoint.  Perhaps the endpoint would not enable ATS if
PCI_ACS_TB is enabled on a parent downstream port.

All of this seems to stem from Cavium incorrectly stealing the ACS bit
mask from the multifunction quirk.  TB is included in that mask because
the spec indicates multifunction devices _must_not_ implement that
capability.  Same for SV and UF, note the comment in
pci_quirk_mf_endpoint_acs().  We include those in order to clear flags
that are either not relevant to those endpoints or we're claiming
are covered by lack or p2p between functions.  It's all a simplication
for the caller.  It's possible that the multifunction quirk exposing DT
is incorrect here in general.  Note that in the call path for these
quirks we're not testing what the device is only capable of, we're
testing what the device has effectively enabled, ie.
pci_dev_specific_acs_enabled().  Thanks,

Alex

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