Re: Query: ARM64: Behavior of el1_dbg exception while executing el0_dbg

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jan 13, 2015 at 06:46:36AM +0000, Pratyush Anand wrote:
> On Monday 12 January 2015 11:00 PM, Will Deacon wrote:
> > On Fri, Jan 09, 2015 at 05:13:29PM +0000, Pratyush Anand wrote:
> >> On Friday 09 January 2015 09:16 PM, Will Deacon wrote:
> >>> On Thu, Jan 08, 2015 at 05:28:37PM +0000, Pratyush Anand wrote:
> >>>> On Thursday 08 January 2015 09:53 PM, Will Deacon wrote:
> >>>>> On Thu, Jan 08, 2015 at 01:15:58PM +0000, Pratyush Anand wrote:
> >>>>>> I am trying to test following scenario, which seems valid to me. But I
> >>>>>> am very new to ARM64 as well as to debugging tools, so seeking expert's
> >>>>>> comment here.
> >>>>>>
> >>>>>> -- I have inserted a kprobe to the function uprobe_breakpoint_handler
> >>>>>> which is called from elo_dbg
> >>>>>> (el0_dbg->do_debug_exception->brk_handler->call_break_hook->uprobe_breakpoint_handler)
> >>>>>>
> >>>>>> -- kprobe is enabled.
> >>>>>>
> >>>>>> -- an uprobe is inserted into a test application and enabled.
> >>>>>>
> >>>>>> So, when uprobe is enabled and test code execution reaches to probe
> >>>>>> instruction, it executes uprobe breakpoint instruction and el0_dbg
> >>>>>> exception is raised.
> >>>>>>
> >>>>>> When control reaches to start of uprobe_breakpoint_handler and it
> >>>>>> executes first instruction (which has been replaced with a kprobe
> >>>>>> breakpoint instruction), el1_dbg exception is raised.
> >>>>>
> >>>>> Hmm, debug exceptions should be masked at this point so I don't see why
> >>>>> you're taking the second debug exception.
> >>>>
> >>>> So, you mean to say that when an exception which has been taken from
> >>>> lower exception level (EL0) is being executed, then we keep masked also
> >>>> the exception from current exception level (EL1)...
> >>>
> >>> Yeah, if you look at entry.S then you'll see that neither el0_dbg or el1_dbg
> >>> re-enable debug exceptions (masked automatically by the CPU after taking the
> >>> exception) until *after* the handling has completed. This is to prevent
> >>> recursive debug exceptions, which I don't see how we can reasonable handle.
> >>
> >> May be I am missing something, but my observation on silicon is
> >> different. Please have a look at git log of HEAD of following branch,
> >> which says that el1_dbg exception has been raised while el0_dbg was
> >> executing. Do not know what I am missing..
[...]
> > Regardless, I think you need to debug further and found out if PSTATE.D is
> > getting cleared and, if so, who is responsible for that. Somebody could be
> > enabling IRQs, for example, which will then unmask debug exceptions in
> > el1_irq.
> 
> This is what I see for pstate, When el0_dbg exception is raised (ie an 
> exception raised with ESR = ESR_EL1_EC_BRK64 after executing instruction 
> BRK64_OPCODE_UPROBES = 0xD4200100 in EL0, user mode), spsr_el1 value is 
> 0x80000000. Which means, all exceptions are unmasked. Is it expected?

spsr_el1 is the EL0 pstate saved when entering EL1. So it is expected
that user space always has interrupts enabled.

-- 
Catalin