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

From: Andy Lutomirski
Date: Sat Mar 12 2016 - 20:03:16 EST

On Fri, Mar 11, 2016 at 2:36 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> On Thu, 2016-03-10 at 22:47 -0800, Andy Lutomirski wrote:
>> On Mon, Mar 7, 2016 at 9:03 AM, Toshi Kani <toshi.kani@xxxxxxx> 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.
> Interesting. I wonder how much we can rely on this feature. Yes, by
> looking at Intel SDM, it is indeed poorly documented. :-(

Any Intel people want to give us a hint?