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[Qemu-devel] software breakpoints disappearing/reappearing in KVM/qemu
From: |
Craig Brozefsky |
Subject: |
[Qemu-devel] software breakpoints disappearing/reappearing in KVM/qemu |
Date: |
Fri, 8 Apr 2011 13:50:35 -0500 |
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.
- [Qemu-devel] software breakpoints disappearing/reappearing in KVM/qemu,
Craig Brozefsky <=