dtrace discuss - Aug 2005 - issues dereferencing memory with dtrace

I''m trying to do something a bit odd with dtrace and can''t
figure out
why this won''t work.  What I''m trying to do for root causing a
bug I''m
seeing is to print out the contents of an OBP address during probes to
see when a specific value gets set.  Via mdb I can do this:
> 0xf067b0c0/X
0xf067b0c0:     1020300         

However, the following (trimmed down) dscript does not work with this
address.  If I chose a known kernel address, I can get it to work, but
not with this OBP address.

#!/usr/sbin/dtrace -Fs
END
{
        trace(*(uint_t *)0xf067b0c0);
}


Is there a known limitation I''m running into here?  Anyone have any
suggestions as to a workaround?

Thanks,

Mike

> I''m trying to do something a bit odd with dtrace and
can''t figure out
> why this won''t work.  What I''m trying to do for root
causing a bug I''m
> seeing is to print out the contents of an OBP address during probes to
> see when a specific value gets set.  Via mdb I can do this:
> 
> > 0xf067b0c0/X
> 0xf067b0c0:     1020300         
> 
> However, the following (trimmed down) dscript does not work with this
> address.  If I chose a known kernel address, I can get it to work, but
> not with this OBP address.
> 
> #!/usr/sbin/dtrace -Fs
> END
> {
>         trace(*(uint_t *)0xf067b0c0);
> }
> 
> 
> Is there a known limitation I''m running into here?  Anyone have
any
> suggestions as to a workaround?
> 
> Thanks,
> 
> Mike
We can''t permit arbitrary dereferencing of OBP addresses because the
kernel can''t know which of them may be memory-mapped device registers.
Any access to i/o space is verboten in DTrace because it can hang the
machine if you have h/w that is either unsafely designed or has
registers with side-effects on read.

If you have a development system and wish to disable the safety
mechanism you can construct your own kernel without the bit of
sun4''s dtrace_toxic_ranges() that refers to OFW_START_ADDR,
OFW_END_ADDR.

-Mike

-- 
Mike Shapiro, Solaris Kernel Development. blogs.sun.com/mws/