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

[Steven Raasch <sraasch@gmail.com>]

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Understood with your KVM/TCG snapshot comment. I thought it was worth a try.

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.

If I slow down the entire QEMU system with my hacks, then my expectation is
that the time for both the VM and the app will slow down similarly (if I
decouple the VM time from real-world time using the -rtc command-line
argument).

So...
   1) Are my assumptions close?
   2) Can someone point me to information on using gdb with QEMU/KVM?

Thanks!
-S




On Thu, Jun 24, 2021 at 11:23 AM Peter Maydell <peter.maydell@linaro.org>
wrote:

> On Wed, 23 Jun 2021 at 22:10, Steven Raasch <sraasch@gmail.com> wrote:
> > I have used KVM to create a snapshot of a windows-10 guest running a
> graphics-intensive app. The *original* issue is that the app does not
> execute correctly when re-started from the snapshot using TCG (it doesn't
> crash, but it doesn't run correctly, either).
>
> I'm not sure that taking a snapshot with KVM and then resuming under TCG
> is really tested. So I'm not very surprised that it doesn't work.
>
> > I'm setting DEBUG & single-step modes by calling cpu_single_step() from
> the top of kvm_vcpu_thread_fn().
> > in kvm_cpu_exec() I wait until I get a KVM_EXIT_DEBUG signal before
> logging the instruction.
>
> If your app can cope with the slowdown involved in taking a VM exit
> after every instruction (which will be massive), then it can probably
> also handle the extra overhead on top of that of the gdbstub communication
> protocol. So it's probably simplest just to connect to QEMU's gdbstub and
> do the single-stepping that way.
>
> The other approach to this would be to see if intel's perf monitor
> stuff (which I know nothing about) has some kind of execution-trace
> capture support and if that works when passing through the PMU to a
> KVM guest.
>
> thanks
> -- PMM
>

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