package/qemu: add option to enable tracing

Tracing is a development feature for debugging, profiling, and observing
QEMU execution. It does not make sense to enable it by default, so add a
config to enable the "log" tracing backend (the default one). Options to
select other backends may be added in the future.

Also pull a patch already reviewed upstream to install the trace events
file only if necessary:

    https://patchwork.kernel.org/project/qemu-devel/patch/20230408010410.281263-1-casantos@redhat.com/

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/qemu/0004-tracing-install-trace-events-file-only-if-necessary.patch

@@ -0,0 +1,30 @@
+From 5c43da1e4983cc3c209b325a5228b6149e0a0ccf Mon Sep 17 00:00:00 2001
+From: Carlos Santos <casantos@redhat.com>
+Date: Fri, 24 Mar 2023 21:40:22 -0300
+Subject: [PATCH] tracing: install trace events file only if necessary
+
+It is not useful when configuring with --enable-trace-backends=nop.
+
+Upstream: https://patchwork.kernel.org/project/qemu-devel/patch/20230408010410.281263-1-casantos@redhat.com/
+Signed-off-by: Carlos Santos <casantos@redhat.com>
+Signed-off-by: Carlos Santos <unixmania@gmail.com>
+---
+ trace/meson.build | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/trace/meson.build b/trace/meson.build
+index 8e80be895c..30b1d942eb 100644
+--- a/trace/meson.build
++++ b/trace/meson.build
+@@ -64,7 +64,7 @@ trace_events_all = custom_target('trace-events-all',
+                                  input: trace_events_files,
+                                  command: [ 'cat', '@INPUT@' ],
+                                  capture: true,
+-                                 install: true,
++                                 install: get_option('trace_backends') != [ 'nop' ],
+                                  install_dir: qemu_datadir)
+ 
+ if 'ust' in get_option('trace_backends')
+-- 
+2.31.1
+

package/qemu/Config.in

@@ -106,6 +106,15 @@ config BR2_PACKAGE_QEMU_FDT
 	  Say 'y' here to have QEMU capable of constructing Device
 	  Trees, and passing them to the VMs.
 
+config BR2_PACKAGE_QEMU_TRACING
+	bool "Enable tracing"
+	help
+	  Say 'y' to enable the "log" tracing infrastructure in QEMU,
+	  used for debugging, profiling, and observing execution. For
+	  detailed documentation, see
+
+	  https://www.qemu.org/docs/master/devel/tracing.html
+
 endif # BR2_PACKAGE_QEMU_SYSTEM
 
 comment "systems emulation needs a toolchain w/ dynamic library"

package/qemu/qemu.mk

@@ -160,6 +160,12 @@ else
 QEMU_OPTS += --disable-fdt
 endif
 
+ifeq ($(BR2_PACKAGE_QEMU_TRACING),y)
+QEMU_OPTS += --enable-trace-backends=log
+else
+QEMU_OPTS += --enable-trace-backends=nop
+endif
+
 ifeq ($(BR2_PACKAGE_QEMU_TOOLS),y)
 QEMU_OPTS += --enable-tools
 else