ACK/Cmnt: [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 3:16 PM Stefan Bader <stefan.bader at canonical.com> wrote:
>
> On 16.07.21 18:58, 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.
> >
> > Kai-Heng Feng (1):
> >    UBUNTU: SAUCE: iommu/amd: Keep swiotlb enabled to ensure devices with
> >      32bit DMA still work
> >
> >   drivers/iommu/amd/iommu.c | 2 --
> >   1 file changed, 2 deletions(-)
> >
>
> Hi Kai,
>
> with all the things going on nowadays it might be best to be more obvious in the
> explanation of SAUCE patches for stable releases (Hirsute in this case).
> Glancing over those submission is often done with little time (and I can only
> speak for myself here, in mornings without enough coffee, yet) so there is often
> a chance to miss things which are not really obvious. So having a paragraph at
> the beginning of the cover email that states "this is temporary while a proper
> fix is investigated" might be better that saying the same somewhere at the
> bottom of the commit message.

OK, I'll always put such notice in first paragraph.

>
> I take it that we still have no proper solution or an ETA for it? So preventing
> crashes surely is worth having a SAUCE patch temporarily (thought band-aids tend
> to last forever). So

I should be able to work on it soon. But there's no ETA to be merged upstream.

Kai-Heng

>
> Acked-by: Stefan Bader <stefan.bader at canonical.com>
>