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

[Pratyush Anand]

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..
> >
> > https://github.com/pratyushanand/linux/tree/ml_arm64_uprobe_devel_debug_kprobe_insertion_at_uprobe_breakpoint_handler
>
> That page just says "Failed to load latest commit information." for me.

may be you can fetch https://github.com/pratyushanand/linux.git and can 
see git log of HEAD of 
ml_arm64_uprobe_devel_debug_kprobe_insertion_at_uprobe_breakpoint_handler.

Or, you can apply attached patches on top of v3.18 kernel.

> 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?


~Pratyush

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