Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for untrusted/external-facing devices

["Raj, Ashok" <ashok.raj@intel.com>]

Hi Bjorn


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.  And (I guess, I'm not an expert) it can
> also never use the Page Request Services?

Yep, sounds like it.

> 
> Is this what we want?  Do we have any idea how many external devices
> this will affect or how much of a performance impact they will see?
> 
> Do we need some kind of override or mechanism to authenticate certain
> devices so they can use ATS and PRI?

Sounds like we would need some form of an allow-list to start with so we
can have something in the interim. 

I suppose a future platform might have a facilty to ensure ATS is secure and
authenticated we could enable for all of devices in the system, in addition
to PCI CMA/IDE. 

I think having a global override to enable all devices so platform can
switch to current behavior, or maybe via a cmdline switch.. as much as we
have a billion of those, it still gives an option in case someone needs it.



> 
> If we do decide this is the right thing to do, I think we need to
> expand the commit log a bit, because this is potentially a significant
> user-visible change.
> 
> >  	pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
> >  }
> >  
> > diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> > index b341628e47527..bb22b46c1d719 100644
> > --- a/drivers/pci/quirks.c
> > +++ b/drivers/pci/quirks.c
> > @@ -4934,6 +4934,13 @@ static void pci_quirk_enable_intel_rp_mpc_acs(struct pci_dev *dev)
> >  	}
> >  }
> >  
> > +/*
> > + * Currently this quirk does the equivalent of
> > + * PCI_ACS_SV | PCI_ACS_RR | PCI_ACS_CR | PCI_ACS_UF
> > + *
> > + * TODO: This quirk also needs to do equivalent of PCI_ACS_TB,
> > + * if dev->external_facing || dev->untrusted
> > + */
> >  static int pci_quirk_enable_intel_pch_acs(struct pci_dev *dev)
> >  {
> >  	if (!pci_quirk_intel_pch_acs_match(dev))
> > @@ -4973,6 +4980,14 @@ static int pci_quirk_enable_intel_spt_pch_acs(struct pci_dev *dev)
> >  	ctrl |= (cap & PCI_ACS_CR);
> >  	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");
> > +	}
> > +
> >  	pci_write_config_dword(dev, pos + INTEL_SPT_ACS_CTRL, ctrl);
> >  
> >  	pci_info(dev, "Intel SPT PCH root port ACS workaround enabled\n");
> > -- 
> > 2.27.0.212.ge8ba1cc988-goog
> >