On 17/03/2016:01:27:26 PM, Pratyush Anand wrote:

@David: This patch was added in v9 and fixup_exception() had been dropped in v9.
Since, dropping of fixup_exception() also caused to fail some systemtap test
cases, so it was added back in v10. I wonder if we really need this patch.
May be you can try to run related test case by dropping this patch. 

Had a closer look to the code, and noticed that fixup_exception() does not have
any role in handling of page fault of copy_to_user(). Then, why do we have the
problem.
Probably, I can see why does not it work. So, when we are single stepping an
instruction and page fault occurs, we will come to el1_da in entry.S. Here, we
do enable_dbg. As soon as we will do this, we will start receiving single step
exception after each instruction (not sure, probably for each alternate
instruction). Since, there will not be any matching single step handler for
these instructions, so we will see warning "Unexpected kernel single-step
exception at EL1". 

So, I think, we should 

(1) may be do not enable debug for el1_da, or
(2) enable_dbg only when single stepping is not enabled, or
(3) or disable single stepping during el1_da execution.

(1) will solve the issue for sure, but not sure if it could be the best choice.

Will, what do you suggest?

~Pratyush