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Re: Recompile QEMU with frame-pointers

From: Peter Maydell
Subject: Re: Recompile QEMU with frame-pointers
Date: Thu, 28 Jan 2021 10:29:56 +0000

On Thu, 28 Jan 2021 at 08:42, Jakob Bohm <jb-gnumlists@wisemo.com> wrote:
> On 2021-01-27 15:14, Salvatore Mazzarino wrote:
> I’m trying to profile my QEMU process but what I get is a stack full of 
> unknown.
> I would then need to recompile QEMU with -fno-omit-frame-pointer.
> Do you know if there is a version already built for that purpose?
> I am not sure, but I suspect that compiler-generated frame pointer
> code would interfere with the TCG compilation of tiny code snippets
> to be pasted together at runtime by the translated code generator.

TCG has not worked by pasting together compiled snippets for
many many years now. Compiling QEMU's C code with a frame
pointer should be fine. You'll find that the JITted code looks
like a single big function (ie there is no internal structure
to it) but the entry-to-JIT prologue code should set up the
frame pointer correctly so the stack frame looks like
   C helper function
   [JITted code]
   C code that called into JITted code

There's no pre-built QEMU that was built with -fno-omit-frame-pointer
because it would be pretty useless for anything except debugging
QEMU itself, and if you're doing that you want to be working with
the QEMU source code rather than a prebuilt binary anyway. You
can easily build one yourself using configure's --extra-cflags

-- PMM

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