Messages in this thread | | | Date | Sat, 19 Dec 2009 00:15:47 +0100 | From | Frederic Weisbecker <> | Subject | Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f |
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On Fri, Dec 18, 2009 at 12:05:03PM -0800, 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.
It looks like in kvm, before entering the guest, we restore its debug registers:
vcpu_enter_guest(): if (unlikely(vcpu->arch.switch_db_regs)) { set_debugreg(0, 7); set_debugreg(vcpu->arch.eff_db[0], 0); set_debugreg(vcpu->arch.eff_db[1], 1); set_debugreg(vcpu->arch.eff_db[2], 2); set_debugreg(vcpu->arch.eff_db[3], 3); }
But what happens to dr6, I don't know.
Adding Avi and Jan in Cc.
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