Messages in this thread | | | Date | Fri, 18 Nov 2016 14:59:04 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state machine |
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On Fri, 18 Nov 2016, Will Deacon wrote: > On Fri, Nov 18, 2016 at 02:42:15PM +0100, Thomas Gleixner wrote: > > On Fri, 18 Nov 2016, Will Deacon wrote: > > > On Fri, Nov 18, 2016 at 02:11:58PM +0100, Thomas Gleixner wrote: > > > > But it's guaranteed that cpuhp_setup_state() will not return before the > > > > callback has been invoked on each online cpu. > > > > > > Ok, that's good. > > > > > > > If cpus are not yet online when that code is invoked, then it's the same > > > > behaviour as before. It will be invoked when the cpu comes online. > > > > > > Just to check, but what stops a CPU from coming online between the call > > > to cpuhp_setup_state and the call to cpuhp_remove_state_nocalls in the > > > case of failure (debug_err_mask isn't empty)? > > > > Indeed! I missed that part. So we still need a get/put_online_cpus() > > protection around all of this. > > Yes, that should do it. > > > Just for curiosity sake. Wouldn't it be simpler and less error prone to > > make the ARM_DBG_READ/WRITE macros use the exception table and handle that > > in the undefined instruction handler to avoid this hook dance? > > That would be an option, but it's only the reset sequence that could > generate this fault so it's simpler to isolate it there.
ARM_DBG_READ/WRITE_SAFE() then for reset_ctrl_regs()
> We'd also have to take into account SMP if we toggle the handler in the > READ/WRITE accessors, since the fault handler framework is system-wide as > opposed to per-cpu. The whole thing is grotty as hell.
The exception table is not toggling anything. It's just providing an entry in the exception tables, which is scanned by fixup_exception(), which then moves PC to the exception code. See __get_user_asm().
So the whole thing becomes:
static int reset_ctrl_regs(unsigned cpu) { .... if (ARM_DBG_READ_SAFE(c1, c5, 4, val)) return -ENODEV; .... return 0; }
All you need is the extra
if (fixup_exception(regs)) return;
in do_undefinstr() like it is there in do_kernel_fault(). No hooks, no scope issues, just works.
I just mention this because that's how x86 implements rdmsr/wrmsr_safe() so it can probe msr access. The difference though it that this results in a #GP and not in #UD, but that's not a show stopper :)
Thanks,
tglx
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