Re: Troubles running Xen on Raspberry Pi 4 with 5.6.1 DomU

[Stefano Stabellini <sstabellini@kernel.org>]

Hi Roman,


In regards to the attached stack trace, nothing rings a bell
unfortunately. I don't know why quirk_usb_early_handoff causes a crash.
It would be useful to add a few printk in quirk_usb_early_handoff to
know where the crash is happening exactly.


In regards to Dornerworks's third patch, it doesn't look like it is
related to the quirk_usb_early_handoff crash. The third patch is
probably not useful anymore because dev->archdata.dev_dma_ops is gone
completely. However, just in case, something like the following would
help recognize if the original bug still persists in newer kernels
somehow:


diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index 6c45350e33aa..61af12d79add 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/arch/arm64/mm/dma-mapping.c
@@ -53,7 +53,9 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
 		iommu_setup_dma_ops(dev, dma_base, size);
 
 #ifdef CONFIG_XEN
-	if (xen_initial_domain())
+	if (xen_initial_domain()) {
+		WARN_ON(dev->dma_ops != NULL);
 		dev->dma_ops = &xen_swiotlb_dma_ops;
+	}
 #endif
 }


On Thu, 30 Apr 2020, Roman Shaposhnik wrote:
> Hi!
> 
> I'm trying to run Xen on Raspberry Pi 4 with 5.6.1 stock,
> upstream kernel. The kernel itself works perfectly well
> on the board. When I try booting it as Dom0 under Xen,
> it goes into a stacktrace (attached).
> 
> Looking at what nice folks over at Dornerworks have previously
> done to make RPi kernels boot as Dom0 I've come across these
> 3 patches:
>     https://github.com/dornerworks/xen-rpi4-builder/tree/master/patches/linux
> 
> The first patch seems irrelevant (unless I'm missing something
> really basic here). The 2nd patch applied with no issue (but
> I don't think it is related) but the 3d patch failed to apply on
> account of 5.6.1 kernel no longer having:
>     dev->archdata.dev_dma_ops
> E.g.
>     https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/arm64/mm/dma-mapping.c?h=v5.6.1#n55
> 
> I've tried to emulate the effect of the patch by simply introducing
> a static variable that would signal that we already initialized
> dev->dma_ops -- but that didn't help at all.
> 
> I'm CCing Jeff Kubascik to see if the original authors of that Corey Minyard
> to see if may be they have any suggestions on how this may be dealt
> with.
> 
> Any advice would be greatly appreciated!
> 
> Thanks,
> Roman.
>