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Re: x86: Hardware breakpoints are not always triggered

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On 01/28, Andrey Wagin wrote:
>
> We use hardware breakpoints in CRIU and we found that sometimes we set
> a break-point, but a process doesn't stop on it.

reproduced, and this certainly looks like kvm bug to me.

> The reproducer uses a different break-point address if it is executed
> with arguments than when it executed without arguments.

IOW, multiple processes running in parallel use the same debug register db0
but different address. And it seems that set_debugreg(address, 0) sometime
doesn't work in the guest kernel.

I think I verified the following:

	- debug registers look always correct as it seen by the guest.
	  I used get_debugreg() to dump them after the task misses bp.

	- do_debug() was not called in this case.

	- finally, it seems that the host has the wrong value in db0
	  set by another process.

	  I modified your test-case so that child2() calls child() when
	  it detects the missed bp, and this does trigger do_debug/etc
	  while it should not.

Oleg.

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Thread

x86: Hardware breakpoints are not always triggered Andrey Wagin <avagin@gmail.com> - 2016-01-28 09:40 +0100
  Re: x86: Hardware breakpoints are not always triggered Jan Kiszka <jan.kiszka@siemens.com> - 2016-01-28 12:10 +0100
  Re: x86: Hardware breakpoints are not always triggered Oleg Nesterov <oleg@redhat.com> - 2016-01-28 22:00 +0100
    Re: x86: Hardware breakpoints are not always triggered Paolo Bonzini <pbonzini@redhat.com> - 2016-01-28 22:30 +0100
    Re: x86: Hardware breakpoints are not always triggered Oleg Nesterov <oleg@redhat.com> - 2016-01-28 23:10 +0100
  Re: x86: Hardware breakpoints are not always triggered Paolo Bonzini <pbonzini@redhat.com> - 2016-01-28 22:40 +0100
    Re: x86: Hardware breakpoints are not always triggered Andrey Wagin <avagin@gmail.com> - 2016-01-28 23:50 +0100
      Re: [CRIU] x86: Hardware breakpoints are not always triggered Andrew Vagin <avagin@virtuozzo.com> - 2016-01-29 23:30 +0100
        Re: [CRIU] x86: Hardware breakpoints are not always triggered Paolo Bonzini <pbonzini@redhat.com> - 2016-01-31 12:10 +0100
          Re: [CRIU] x86: Hardware breakpoints are not always triggered Nadav Amit <nadav.amit@gmail.com> - 2016-02-03 19:20 +0100
            Re: [CRIU] x86: Hardware breakpoints are not always triggered Nadav Amit <nadav.amit@gmail.com> - 2016-02-04 00:40 +0100
              Re: [CRIU] x86: Hardware breakpoints are not always triggered Paolo Bonzini <pbonzini@redhat.com> - 2016-02-04 12:30 +0100

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