Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

[Jan Kiszka]

Oleg Nesterov wrote:
> On 12/18, Roland McGrath wrote:
>>> Please find the trivial test-case below. It hangs, because
>>> PTRACE_SINGLESTEP doesn't trigger the trap.
>> 2.6.33-rc1 x86-64 works for me with either -m64 or -m32 version of that test.
>>
>>> (not sure this matters, but I did the testing under kvm)
>> Apparently it does.  You should hack some printks into do_debug() and see
>> how kvm is differing from real hardware.  (Actually you can probably do
>> this with a notifier added by a module, not that you are shy about
>> recompiling!)
>>
>> Probably kvm's emulation of the hardware behavior wrt the DR6 bits is not
>> sufficiently faithful.  Conceivably, kvm is being consistent with some
>> older hardware and we have encoded assumptions that only newer hardware
>> meets.  But I'd guess it's just a plain kvm bug.
> 
> OK, thanks.
> 
> Hmm. Now I see how wrong I was when I said this code is "obviously wrong" ;)
> 
> I'll add the debugging printk's and report the output. Sorry for delay,
> can't do this today.

Can't reproduce, runs fine here with with 2.6.33-rc1 as both host&guest
and qemu-kvm latest git. Host uses kvm-intel.

Can you specify your setup in more details? Which host kernel did you
use, which qemu-kvm version? Are you on AMD or Intel? Any specific guest
kernel config switch that may influence this?

Jan

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