linux-kernel - Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

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Date:	Fri, 18 Dec 2009 23:32:30 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	utrace-devel@...hat.com
Subject: Re: x86: do_debug && PTRACE_SINGLESTEP broken by
	08d68323d1f0c34452e614263b212ca556dae47f

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.

Oleg.

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