Re: [PATCH 1/1] iommu/vt-d: Prevent boot failure with devices requiring ATS

[Baolu Lu]

On 2024/9/4 14:49, Tian, Kevin wrote:
> > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Wednesday, September 4, 2024 2:07 PM
> >
> > SOC-integrated devices on some platforms require their PCI ATS enabled
> > for operation when the IOMMU is in scalable mode. Those devices are
> > reported via ACPI/SATC table with the ATC_REQUIRED bit set in the Flags
> > field.
> >
> > The PCI subsystem offers the 'pci=noats' kernel command to disable PCI
> > ATS on all devices. Using 'pci=noat' with devices that require PCI ATS
> > can cause a conflict, leading to boot failure, especially if the device
> > is a graphics device.
> >
> > To prevent this issue, check PCI ATS support before enumerating the IOMMU
> > devices. If any device requires PCI ATS, but PCI ATS is disabled by
> > 'pci=noats', switch the IOMMU to operate in legacy mode to ensure
> > successful booting.
>
> I guess the reason of switching to legacy mode is because the platform
> automatically enables ATS in this mode, as the comment says in
> dmar_ats_supported(). This should be explained otherwise it's unclear
> why switching the mode can make ATS working for those devices.

Not 'automatically enable ATS,' but hardware provides something that is
equivalent to PCI ATS. The ATS capability on the device is still
disabled. That's the reason why such device must be an SOC-integrated
one.

> But then doesn't it break the meaning of 'pci=noats' which means
> disabling ATS physically? It's described as "do not use PCIe ATS and
> IOMMU device IOTLB" in kernel doc, which is not equivalent to
> "leave PCIe ATS to be managed by HW".

Therefore, the PCI ATS is not used and the syntax of pci=noats is not
broken.

> and why would one want to use 'pci=noats' on a platform which
> requires ats?

We don't recommend users to disable ATS on a platform which has devices
that rely on it. But nothing can prevent users from doing so. I am not
sure why it is needed. One possible reason that I can think of is about
security. Sometimes, people don't trust ATS because it allows devices to
access the memory with translated requests directly without any
permission check on the IOMMU end.

Thanks,
baolu