Re: Overlapping ioremap() calls, set_memory_*() semantics

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Mar 7, 2016 at 9:03 AM, Toshi Kani <toshi.kani@hpe.com> wrote:
> Let me try to summarize...
>
> The original issue Luis brought up was that drivers written to work with
> MTRR may create a single ioremap range covering multiple cache attributes
> since MTRR can overwrite cache attribute of a certain range.  Converting
> such drivers with PAT-based ioremap interfaces, i.e. ioremap_wc() and
> ioremap_nocache(), requires a separate ioremap map for each cache
> attribute, which can be challenging as it may result in overlapping ioremap
> ranges (in his term) with different cache attributes.
>
> So, Luis asked about 'sematics of overlapping ioremap()' calls.  Hence, I
> responded that aliasing mapping itself is supported, but alias with
> different cache attribute is not.  We have checks in place to detect such
> condition.  Overlapping ioremap calls with a different cache attribute
> either fails or gets redirected to the existing cache attribute on x86.

A little off-topic, but someone reminded me recently: most recent CPUs
have self-snoop.  It's poorly documented, but on self-snooping CPUs, I
think that a lot of the aliasing issues go away.  We may be able to
optimize the code quite a bit on these CPUs.

I also wonder whether we can drop a bunch of the memtype tracking.
After all, if we have aliases of different types on a self-snooping
CPU and /dev/mem is locked down hard enough, we could maybe get away
with letting self-snoop handle all the conflicts.

(We could also make /dev/mem always do UC if it would help.)

--Andy