Re: xen-swiotlb issue when NVMe driver is enabled in Dom0 on ARM

[Christoph Hellwig <hch@xxxxxx>]

On Thu, Apr 14, 2022 at 01:39:23PM -0700, Stefano Stabellini wrote:
> OK, now we know that the code path with Xen is correct and it is the
> same code path taken (dma_alloc_direct) as when !CONFIG_XEN and !SMMU.
> That is how it should be.
> 
> I cannot explain why dma_alloc_direct() would fail when called from
> xen_swiotlb_alloc_coherent(), but it would succeed when called from
> dma_alloc_attrs() without Xen.
> 
> I am not aware of any restrictions that xen or swiotlb-xen would
> introduce in that regard. Unless you are just running out of memory
> because dom0_mem too low.

The crash is deep down in the page allocator.  Even if memory was low
it should no crash.  So there is some odd interaction between Xen
and the page allocator going on.  I think nvme and dma-direct really
are only the messenger here.