x86 swiotlb questions

["Jan Beulich" <jbeulich@novell.com>]

I'm not certain when this code was changed significantly from what I remember, but

- What is the purpose of using alloc_bootmem_low variants here? I.e., where is the
dependency on physical addresses being below 4G here (machine addresses are
being restricted after the allocation anyway)? The panic message text after the
failed allocation is confusing me additionally.

- While I can see the idea behind the overflow buffer, it doesn't seem to prevent
data corruption, and if I understand it correctly it doesn't even prevent memory
corruption (since its machine address doesn't get restricted anywhere, so the fall
back return value would not necessarily meet the device requirements).

- With various parameters now being command line configurable, if any of these
get set inconsistently or incorrectly the user would probably get a cryptic crash
(from the BUG_ON() following the call to xen_create_contiguous_region()).

- The default bit width for DMA (also in Xen itself) was now changed to 30, just
because of a single device (b44). Shouldn't it, with the settings being
customizable now, rather be 32 (and those who own such ill devices need to
make use of the option)?

- The DMA bit widths can be set to different values in Xen and kernel, which can
lead to surprising results, I would think. Shouldn't the kernel rather obtain Xen's
value, so they are consistent?

Thanks, Jan