Re: Extracting PC information from QEMU/KVM during single-step

[Steven Raasch <sraasch@gmail.com>]

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Peter -

Thanks for the clarification, that makes perfect sense.

In this case, using GDB is clearly the best way to go about the tracing.

I appreciate your time!

-S

On Thu, Jun 24, 2021 at 1:51 PM Peter Maydell <peter.maydell@linaro.org>
wrote:

> On Thu, 24 Jun 2021 at 18:09, Steven Raasch <sraasch@gmail.com> wrote:
> > NOTE: I do not yet understand how gdb interacts with the virtual
> machine. I have experience with GDB, but only at a linux app-debug level. I
> don't grok how gdb on a linux host works with QEMU running a windows guest.
> > My *assumption* is that the VM continues to run while an app is being
> debugged with GDB can be stopped, stepped, etc. If this is the case, I
> would expect that the VM's sense of time will continue to move forward
> while the app is paused. This would be an issue for my time-sensitive app.
>
> No, that's not how it works. A gdb connected to QEMU's gdbstub is a
> bit like a hardware JTAG debugger connected to a real CPU, if that
> helps. When gdb gets control the entire VM is stopped; stepping
> steps one instruction of whatever the VM is doing. gdb and the
> gdbstub have no understanding of individual processes running
> inside the guest OS -- single stepping will happily step through
> the app, into interrupt handlers, across the OS context switching
> and into other processes, etc.
>
> -- PMM
>

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