Hi James,

On 18/03/2016:02:02:49 PM, James Morse wrote:

Hi Pratyush,

On 18/03/16 13:29, Pratyush Anand wrote:

quoted

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.

A variation on (3):

In kernel/entry.S when entered from EL0 we test for TIF_SINGLESTEP in the
thread_info flags, and use disable_step_tsk/enable_step_tsk to save/restore the
single-step state.

Could we do this regardless of which EL we came from?

Thanks for another idea. I think, we can not do this as it is, because
TIF_SINGLESTEP will not be set for kprobe events. But, we can introduce a
variant disable_step_kernel and enable_step_kernel, which can be called in
el1_da. 

I will write a test case to reproduce the issue without this patch, and then
will do test with a patch based on something like above.

~Pratyush