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

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Message-Id: <20091218200503.2CA0DF9A4@magilla.sf.frob.com>
Date:	Fri, 18 Dec 2009 12:05:03 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...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

> 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.

Thanks,
Roland
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