[Qemu-devel] software breakpoints disappearing/reappearing in KVM/qemu

[Craig Brozefsky <craig@red-bean.com>]

First, thank you very much for your work on KVM/QEMU and in particular
the GDB support.  I have been a heavy user of it while implementing a
programmable debugger that monitors/traces guest operating systems.  I
need more breakpoints than hardware BPs available, and I need the
performance of KVM acceleration.

I am running into a problem with software breakpoints disappearing and
re-appearing.  I set them via the GDB stub.  If I examine the location
in the qemu monitor I will see them there, but on occasion (and with
greater frequency the more breakpoints I have) a given breakpoint will
disappear according to the monitor, and then re-appear.

I also have processes executing the instructions at the breakpoint without
stopping.  This makes my results quite unpredictable, and it has led too a
few days of learning all about Windows system working set paging.

My first approach was to try and re-install the breakpoints after a
page was faulted in, thinking that the INT3 was disappearing when the
page was read back into memory.  Setting a hbreak, I was trapping
MMAccessFault, and reinstalling all by breakpoints when it hit its
return address (after the IO Page Read Calls).  That didn't solve
the problem, and left me wondering if I need to take into consideration the way
KVM/qemu handle guest memory.  And that brings me to my question.

Assuming I handle the guest kernel paging issue properly, are you
aware of issues with the debugging support in KVM/qemu that would
result in the INT3 blinking in and out of existence?

In the meantime, I've been reading source code, but it'll be a few
days before I figure out the details of memory mgmt under KVM.  Any
hints would be greatly appreciated.

Thank you for your time.