On 2-Jun-13, at 1:34 PM, James Bottomley wrote:
On Sun, 2013-06-02 at 13:16 -0400, John David Anglin wrote:
On 2-Jun-13, at 12:43 PM, James Bottomley wrote:
On Sun, 2013-06-02 at 12:21 -0400, John David Anglin wrote:
The comment at the start of pacache.S states that the base and
index
registers used for
fdc,fic, and pdc instructions should not use shadowed registers.
Although this is probably
unnecessary for tmpalias flushes, there is also no reason not to
comply. The same
index register (%r23) is used as in other routines.
Please don't do this, it's a misinterpretation of the comment.
I don't think so. See the discussion around cases 16 and 17 in
handle_interruption.
Please be clearer: I don't understand what you think the problem
is. 16
and 17 are the non access TLB traps which we can't get into in the
tmpalias code.
The way we handle tlb misses (for both NA, D and I types) within the
tmpalias region is in the do_alias macro of the interruption handler.
We never go through to the fault handler.
What the comment is trying to say is that we use unshadowed
registers to
pass information to the tmpalias handler in our tlb insertion
interruption. It's the do_alias macro in entry.S. The only actual
unshadowed registers it uses are %r23 and %r26. Apart from these
having
to have specific values, the rest of the routine is free to use any
other registers (either shadowed or unshadowed) as it sees fit.
If the comment is unclear, perhaps it needs fixing?
%r23 is only used by the do_alias macro for interruptions in the
'from' region.
See this hunk:
#ifdef CONFIG_64BIT
extrd,u,*= \va,41,1,%r0
#else
extrw,u,= \va,9,1,%r0
#endif
or,COND(tr) %r23,%r0,\pte
or %r26,%r0,\pte
The extract instruction tests whether the interruption was in the
'to'
tmpalias
region. If it is in the 'to' region, the instruction following using
%r23 is nullified.
So, %r23 can be used for the index register in these two functions
which don't
use the 'from' tmpalias region.
It can, but it's dangerous and confusing to use it because we've
specifically called out that it has a special meaning in the vario0us
comments. Using it for a different purpose from what we stated in the
comments within the code needlessly confuses the reader who comes
after
us.
I don't think it's dangerous. %r23 is a call clobbered register in
these functions.
It can be anything on entry to these functions and it is not
initialized.
We could use %r31 instead if you want to keep %r23 for tmpalias use.
The issue is the inconsistency between the initial comment which
applies to the
fault handler and the tmpalias routines that have different rules for
non access
faults.
Helge originally pointed out the issue.
Dave
--
John David Anglin dave.anglin@xxxxxxxx
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