Re: [PATCH v2 2/2] PCI: Add the enhanced ACS controls check to pci_acs_flags_enabled()

[Wei Wang]

On 1/27/26 2:01 AM, Jason Gunthorpe wrote:
> On Mon, Jan 26, 2026 at 05:10:18PM +0800, Wei Wang wrote:
> > On 1/23/26 10:51 PM, Jason Gunthorpe wrote:
> > > On Fri, Jan 23, 2026 at 09:49:43AM +0800, Wei Wang wrote:
> > > > The enhanced ACS controls introduced by PCIe Gen 5 ensures better device
> > > > isolation. On devices that support the PCI_ACS_ECAP capability, the
> > > > controls are required to be enabled properly:
> > > > - ACS I/O Request Blocking needs to be enabled to avoid unintended
> > > >     upstream I/O requests.
> > > > - ACS DSP and USP Memory Target Access Control needs to be set with
> > > >     Request Redirect or Request Blocking to ensure the Downstream and
> > > >     and Upstream Port memory resource ranges are not accessed by upstream
> > > >     memory requests.
> > > > - ACS Unclaimed Request Redirect needs to be enabled to ensure accesses to
> > > >     areas that lies within a Switch's Upstream Port memory apertures but not
> > > >     within any Downstream Port memory apertures get redirected.
> > > >
> > > > To maintain compatibility with legacy devices that lack PCI_ACS_ECAP
> > > > support, pci_acs_enabled() skips checking for the capability and logs a
> > > > warning to indicate that isolation may be incomplete.
> > >
> > > That's every existing system, please don't do that.
> > >
> > > The issue with ECAP is the way PCI SIG re-defined what Linux has been
> > > doing forever as unsafe.
> >
> > My viewpoint is that there are known bugs with the legacy ACS defined in
> > PCIe Gen 4, and PCIe Gen 5 attempts to address them through new controls in
> > the Enhanced Capability (ECAP). From the Linux perspective, we just need to
> > adapt accordingly to ensure 'better' isolation.
>
> No, there are not "bugs", there is a gap where a very rare and narrow
> HW configuration was not specified. The vast majority of systems don't
> even hit this gap, and when they do switches that are safe and secure
> to use with Linux do the right thing..
>
> > The warning is intended to raise awareness for users, so they can make
> > informed decisions about continuing with this setup.
>
> Then print warnings only in known bad cases by detecting the holes and
> evaluating the quirks database if the device malfunctions. That's
> hard, so I would just skip the print..

OK, I'll drop the warning.