Re: x86: Hardware breakpoints are not always triggered

From: Oleg Nesterov
Date: Thu Jan 28 2016 - 15:53:38 EST


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.