Re: Uncached memory allocator for ia64.

[Robin Holt <holt@sgi.com>]

On Fri, Sep 17, 2004 at 09:34:58AM -0500, Robin Holt wrote:
> 
> I have done a little testing on the uncached.  I think the problem
> may be bigger than I originally expected.

> I made a simple driver.  On load, it allocated an entire granule and,
> I think, correctly did all the flushes called for in the processor
> manual including the PAL call.  A user could then mmap the entire
> chunk as uncached and work with it.  I did not get any sort of MCAs
> from this run.

To be a little more specific, I was in section 4.4.6.1 Disabling Prefetch
and Removing Cacheability.  Jack Steiner made a comment to the effect
of there were additional steps that he knew someone had determined
were necessary.  Unfortunately, he is on vacation now.

> 
> I then started the same app which referenced the first word of each page
> uncached.  I added a timer interrupt which scanned all the page structs
> on the node from which the granule was allocated and had a reference
> to the page inside of an impossible if statement (next to impossible
> as the machine would have to be up for a large number of years).
> This, I believe, resulted in the speculation of the cache line dirty.
> By running this test for about 8 minutes, I was able to cause an MCA
> due to having both a cached and uncached reference to the same cache
> line on the FSB.

> Note, I was running all the pages structs for the node and not just
> the ones for this granule.

> Based on this test, I was wondering if it is safe to reuse a granule
> and leave the page structs in place.  Is this test representative
> of events which could happen?  Can we destroy the page structs on a
> running system?

> Thank you in advance for any direction anybody can give me.

I am not sure what will be acceptable at this point.  Should I write
an uncached allocator which grabs the granules at boot time before they
are ever initialized for cacheable use?  If so, would it be acceptable
to just shrink each efi memory map entry by a command line specified
size during the efi_memmap_walk callout?  At this point I am so vague
on what I should be doing that I am afraid to do much of anything.

Thanks again,
Robin Holt