linux-kernel - Re: [PATCH V2] pci: quirk: Apply APM ACS quirk to XGene devices

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Date:   Mon, 24 Jul 2017 10:33:25 -0700
From:   Feng Kan <fkan@....com>
To:     Alex Williamson <alex.williamson@...hat.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 V2] pci: quirk: Apply APM ACS quirk to XGene devices

On Sun, Jul 23, 2017 at 7:06 PM, Alex Williamson
<alex.williamson@...hat.com> wrote:
> On Fri, 21 Jul 2017 13:20:18 -0700
> Feng Kan <fkan@....com> wrote:
>
>> On Thu, Jul 20, 2017 at 3:22 PM, Alex Williamson
>> <alex.williamson@...hat.com> wrote:
>> > On Wed, 19 Jul 2017 17:46:51 -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. Turn on ACS but disable all the peer to peer features.
>> >>
>> >> Signed-off-by: Feng Kan <fkan@....com>
>> >> ---
>> >>  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 }
>> >>  };
>> >>
>> >
>> > Sorry, I'm not yet convinced there's an equivalent of SV, if a device
>> > spoofs a different bdf and it reaches the smmu, what prevents that from
>> > simply referencing the context for that alternate bdf?
>> Perhaps I am not understanding the question correctly. The bdf forms a
>> stream id which is used to provide an context. Since there is no actual
>> context created by an alternate bdf, the transaction would be rejected
>> by the SMMU.
>
> Unless that context does exist.  Take for example a basic topology:
>
> -[0000:00]-+-00.0
>            +-01.0-[01]----00.0
>            +-02.0-[02]----00.0
>
> Assume 00:01.0 and 00:02.0 are root ports and 01:00.0 and 02:00.0 are
> their respective downstream endpoint.  A single iommu would typically
> handle both of these endpoints using the requester ID, aka stream ID, to
> reference the appropriate context.  Source validation at the root port
> makes sure that any forwarded transaction has a requester ID that falls
> between the secondary and subordinate bus number range of the root
> port.  For instance, if device 01:00.0 was able to generate a
> transaction with a requester ID of 02:00.0, source validation at the
> root port would generate an ACS violation.  If the root port does not
> support source validation, the transaction might successfully reference
> the iommu context for the other endpoint.  Therefore I don't understand
> what property of the SMMU is able to prevent this spoofing technique if
> the root port provides no protection on its own.  Thanks,
Yes, you are right. I left out some details to our implementation. Our
implementation
of the stream ID is not just the BDF. The stream ID also contains a
port ID at the
upper bits to indicate the difference between the PCIe ports. The scenario you
explained up above should not happen.
>
> Alex