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

[Stefan Bader]

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.

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

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

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