[SRU] [H/I/Unstable/OEM-5.10/OEM-5.13] [PATCH 0/1] Fix kernel panic caused by legacy devices on AMD platforms

[Kai-Heng Feng]

On Thu, Aug 19, 2021 at 3:32 PM Andrea Righi <andrea.righi at canonical.com> wrote:
>
> On Sat, Jul 17, 2021 at 12:58:02AM +0800, Kai-Heng Feng wrote:
> > BugLink: https://bugs.launchpad.net/bugs/1936682
> >
> > [Impact]
> > When a legacy device is only 32bit DMA capable and it's in the same
> > IOMMU group with iommu_v2 capable devices, the device in question will
> > be forced to use identity mapping and triggers kernel panic on DMA
> > operation because it can't do 64bit DMA.
> >
> > [Fix]
> > Keep swiotlb enabled so legacy devices can do 64bit DMA. This is also
> > how Intel and ARM64 platforms deal with legacy devices.
> >
> > [Test]
> > Boot an affected system. Kernel panic in Realtek WiFi driver's probe
> > routine.
> > After the patch is applied, the system can work normally.
> >
> > [Where problems could occur]
> > The default swiotlb uses 64MB memory, so if the system doesn't have any
> > legacy device, there are 64MB ram less for the system to use.
>
> Isn't this going to affect other drivers/systems potentially? Keeping
> the swiotlb enabled is going to add only the extra memory consumption
> (64MB) or is it going to potentially affect also the performance of some
> devices?

It's only going to affect devices that can do 32bit DMA, it's either
use swiotlb or kernel panic.

>
> Moreover, even if 64MB doesn't seem that much, it could be a significant
> amount of memory in small systems (embedded, or potentially even VMs
> with a limited amount of memory maybe).

Well, Intel and ARM64 always eat 64MB right now...
For the other case, does guest VM need IOMMU DMA translation? I am not
sure but I thought that's done in the host.

>
> Would it be possible to force swiotlb to be enabled only when some of
> these legacy 32-bit only DMA devices are present?

Yes, that's what maintainer suggested.
But since iommu type can be changed at runtime, the fix isn't that trivial.

Kai-Heng

>
> Thanks,
> -Andrea