"swiotlb buffer is full" in DomU with PCI passthrough is caused by low memory in Dom0

[Paul Leiber <paul@xxxxxxxxxxxxxxxx>]

Hi,

this message is meant mainly to document an issue and the solution to 
help others in a similar situation.
I came across this situation when installing Xen on a small PC (PC 
Engines APU4D4) with 4 GB of RAM and passing through a PCI device (a 
Compex WLE900VX wifi card) to a OpenWRT DomU. In the beginning, I wanted 
to use as little memory as possible for the Debian Dom0, therefore I set 
its memory to 512 MB by using "dom0_mem=512M,max:512M". When trying to 
pass through the PCI card to the DomU, the DomU could see it, but it did 
not work. Dmesg output in the OpenWRT DomU showed several instances of 
the following line:
"ath10k_pci 0000:00:00.0: swiotlb buffer is full (sz: 2112 bytes), total 
2048 (slots), used 2034 (slots)"
Everything else in DomU seemed to work. Searching the web for this error 
message did not give me an indication for the cause. I also could not 
find any cause in the Dom0 logs. Therefore, I could not find the reason 
for this issue for quite some time.
I then randomly tried out to enlarge Dom0 memory to 768 MB, and the 
error message in the DomU disappeared during the next boot. The PCI 
device was working.
I can't rule out that there actually is a log entry in Dom0 for this 
situation, and my Xen and/or linux knowledge was not good enough to let 
me find it. If this is the case, I'd appreciate some pointers.
Best regards,

Paul