Re: [PATCH] pci: quirk: Apply APM ACS quirk to XGene devices

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 18 Jul 2017 10:42:40 -0700
Feng Kan <fkan@apm.com> wrote:

> On Mon, Jul 17, 2017 at 7:23 PM, Alex Williamson
> <alex.williamson@redhat.com> wrote:
> > On Mon, 17 Jul 2017 17:45:52 -0700
> > Feng Kan <fkan@apm.com> wrote:
> >  
> >> The APM X-Gene PCIe root port does not support ACS at this point.
> >> Since the root does not allow peer to peer transactions, mask out
> >> ACS capability flag bits.
> >>
> >> Signed-off-by: Feng Kan <fkan@apm.com>
> >> ---
> >>  drivers/pci/quirks.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> >> index 085fb78..951064d 100644
> >> --- a/drivers/pci/quirks.c
> >> +++ b/drivers/pci/quirks.c
> >> @@ -4368,6 +4368,8 @@ 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_mf_endpoint_acs },
> >>       { 0 }
> >>  };  
> >
> >
> > You're using the "mf_enpoint" quirk for something that is not an
> > endpoint and may or may not be multi-function.  Downstream ports have
> > different rules than multi-function endpoints and the quirk function is
> > commented to reflect why certain fields are relevant or not to
> > multi-function endpoints.  I don't think it's valid to use this quirk
> > for other device types, we'd only need to accidentally test the assumed
> > device type in that function to break you in the future.  
> Would creating another function like what Cavium did be sufficient?

A function that exposes the actual isolation capabilities of the
hardware would be better, yes.

> Additionally,
> > lack of peer to peer does not necessarily imply functionality like
> > Source Validation, without which a device can spoof the requester ID.
> > Thanks,  
> I can remove the statement.

I don't know what that means, does the hardware support an equivalent
to source validation or not?  What's the response of the root port if
the downstream device issues a transaction spoofing devices not within
the bus number ranges of the bridge?

> Alex, the goal here is to enable virtualization to work correctly.
> Please let me know if the
> above is sufficient. Much thanks.

Of course, but that means that the hardware vendor is vouching that
this device provides the equivalent isolation for each of the missing
components of ACS.  Claiming to have isolation capabilities that don't
exist would be irresponsible and put users of that hardware at risk.
Thanks,

Alex