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

[Andrea Righi]

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?

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).

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

Thanks,
-Andrea