Hello,
This is my first post to the linuxppc-embedded list. Please forgive me
for jumping in late onto the CPU15 workaround discussions, but my
company is very interested in finding an efficient fix for this silicon
errata on our devices.
I applied the patch submitted by Scott Wood to our patched denx 2.4.24
tree and did some timing measurements to compare its effect on
performance. We noted a worst case 37% slowdown -- this was during
program load, where the PC is moving all over the place, so the penalty
for all the tlbie's is felt heavily. This is solely anecdotal, I don't
mean to imply that this fix will result in any particular slowdown on
anyone else's systems.
I would like to start a discussion on the possibility of implementing a
selective invalidation of pages on i-tlb miss. On July 20th, 2007 Scott
Wood wrote:
>The only lower-overhead workaround I know of requires compiler
modifications (and I made it configurable to allow for that possibility).
So there likely are reasons why the following is not possible:
In the errata document from Freescale
(http://www.freescale.com/files/32bit/doc/errata/MPC860CE.pdf?fpsp=1),
they list an alternative to the "invalidate previous and next page" itlb
miss handler (see pg 57), where they use the SPS/SH/CI bits of a L2 page
entry to indicate whether: 1) a page is unchecked, 2) it does not
contain a bad branch, 3) it does contain a bad branch, or 4) it has a
previous page with a bad branch. These bits are zeroed before being
written to MI_RPN. In loose terms the itlb miss handle does the
following: If the page is unchecked, they check the last instruction the
current page or the previous page and write the status bits to the L2
entry accordingly. If the page has been checked the status bits are used
to selectively invalidate the next or previous page as needed.
Since MI_RPN is always written with bits 24,25,26 and 27 set; could we
use bits 24, 25 and 26 in memory to indicate the 4 possible page states
and perform selective page invalidation as demonstrated in their example
listing?
Regards,
Ben Gardiner
Nanometrics Inc.
250 Herzberg Rd.
Kanata ON
K2K 2A1
Telephone 613 592 6776 ext 239
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