[PATCH] coroutine: add libucontext as external library

[Joelle van Dyne <j@getutm.app>]

iOS does not support ucontext natively for aarch64 and the sigaltstack is
also unsupported (even worse, it fails silently, see:
https://openradar.appspot.com/13002712 )

As a workaround we include a library implementation of ucontext and add it
as a build option.

Signed-off-by: Joelle van Dyne <j@getutm.app>
---
 configure                 | 21 ++++++++++++++++++---
 meson.build               | 12 +++++++++++-
 util/coroutine-ucontext.c |  9 +++++++++
 .gitmodules               |  3 +++
 MAINTAINERS               |  6 ++++++
 meson_options.txt         |  2 ++
 subprojects/libucontext   |  1 +
 7 files changed, 50 insertions(+), 4 deletions(-)
 create mode 160000 subprojects/libucontext

diff --git a/configure b/configure
index 34fccaa2ba..5f225894a9 100755
--- a/configure
+++ b/configure
@@ -1773,7 +1773,7 @@ Advanced options (experts only):
   --oss-lib                path to OSS library
   --cpu=CPU                Build for host CPU [$cpu]
   --with-coroutine=BACKEND coroutine backend. Supported options:
-                           ucontext, sigaltstack, windows
+                           ucontext, libucontext, sigaltstack, windows
   --enable-gcov            enable test coverage analysis with gcov
   --disable-blobs          disable installing provided firmware blobs
   --with-vss-sdk=SDK-path  enable Windows VSS support in QEMU Guest Agent
@@ -4517,12 +4517,27 @@ else
       error_exit "only the 'windows' coroutine backend is valid for Windows"
     fi
     ;;
+  libucontext)
+  ;;
   *)
     error_exit "unknown coroutine backend $coroutine"
     ;;
   esac
 fi
 
+case $coroutine in
+libucontext)
+  git_submodules="${git_submodules} subprojects/libucontext"
+  mkdir -p libucontext
+  coroutine_impl=ucontext
+  libucontext="enabled"
+  ;;
+*)
+  coroutine_impl=$coroutine
+  libucontext="disabled"
+  ;;
+esac
+
 if test "$coroutine_pool" = ""; then
   coroutine_pool=yes
 fi
@@ -5858,7 +5873,7 @@ if test "$qom_cast_debug" = "yes" ; then
   echo "CONFIG_QOM_CAST_DEBUG=y" >> $config_host_mak
 fi
 
-echo "CONFIG_COROUTINE_BACKEND=$coroutine" >> $config_host_mak
+echo "CONFIG_COROUTINE_BACKEND=$coroutine_impl" >> $config_host_mak
 if test "$coroutine_pool" = "yes" ; then
   echo "CONFIG_COROUTINE_POOL=1" >> $config_host_mak
 else
@@ -6421,7 +6436,7 @@ NINJA=$ninja $meson setup \
         -Dlibnfs=$libnfs -Diconv=$iconv -Dcurses=$curses -Dlibudev=$libudev\
         -Drbd=$rbd -Dlzo=$lzo -Dsnappy=$snappy -Dlzfse=$lzfse \
         -Dzstd=$zstd -Dseccomp=$seccomp -Dvirtfs=$virtfs -Dcap_ng=$cap_ng \
-        -Dattr=$attr -Ddefault_devices=$default_devices \
+        -Dattr=$attr -Ddefault_devices=$default_devices -Ducontext=$libucontext \
         -Ddocs=$docs -Dsphinx_build=$sphinx_build -Dinstall_blobs=$blobs \
         -Dvhost_user_blk_server=$vhost_user_blk_server -Dmultiprocess=$multiprocess \
         -Dfuse=$fuse -Dfuse_lseek=$fuse_lseek -Dguest_agent_msi=$guest_agent_msi \
diff --git a/meson.build b/meson.build
index adeec153d9..2440d90734 100644
--- a/meson.build
+++ b/meson.build
@@ -1633,9 +1633,18 @@ if not fdt.found() and fdt_required.length() > 0
   error('fdt not available but required by targets ' + ', '.join(fdt_required))
 endif
 
+ucontext = dependency('libucontext', kwargs: static_kwargs, required : false)
+if not ucontext.found() and get_option('ucontext').enabled()
+  libucontext_proj = subproject('libucontext',
+                                default_options: ['default_library=static',
+                                                  'freestanding=true'])
+  ucontext = libucontext_proj.get_variable('libucontext_dep')
+endif
+
 config_host_data.set('CONFIG_CAPSTONE', capstone.found())
 config_host_data.set('CONFIG_FDT', fdt.found())
 config_host_data.set('CONFIG_SLIRP', slirp.found())
+config_host_data.set('CONFIG_LIBUCONTEXT', ucontext.found())
 
 #####################
 # Generated sources #
@@ -1883,7 +1892,7 @@ util_ss.add_all(trace_ss)
 util_ss = util_ss.apply(config_all, strict: false)
 libqemuutil = static_library('qemuutil',
                              sources: util_ss.sources() + stub_ss.sources() + genh,
-                             dependencies: [util_ss.dependencies(), m, glib, socket, malloc])
+                             dependencies: [util_ss.dependencies(), m, glib, socket, malloc, ucontext])
 qemuutil = declare_dependency(link_with: libqemuutil,
                               sources: genh + version_res)
 
@@ -2579,6 +2588,7 @@ summary(summary_info, bool_yn: true, section: 'Targets and accelerators')
 
 # Block layer
 summary_info = {}
+summary_info += {'libucontext support': ucontext.found()}
 summary_info += {'coroutine backend': config_host['CONFIG_COROUTINE_BACKEND']}
 summary_info += {'coroutine pool':    config_host['CONFIG_COROUTINE_POOL'] == '1'}
 if have_block
diff --git a/util/coroutine-ucontext.c b/util/coroutine-ucontext.c
index 904b375192..220c57a743 100644
--- a/util/coroutine-ucontext.c
+++ b/util/coroutine-ucontext.c
@@ -23,7 +23,16 @@
 #undef _FORTIFY_SOURCE
 #endif
 #include "qemu/osdep.h"
+#if defined(CONFIG_LIBUCONTEXT)
+#include <libucontext/libucontext.h>
+#define ucontext_t libucontext_ucontext_t
+#define getcontext libucontext_getcontext
+#define setcontext libucontext_setcontext
+#define swapcontext libucontext_swapcontext
+#define makecontext libucontext_makecontext
+#else
 #include <ucontext.h>
+#endif
 #include "qemu/coroutine_int.h"
 
 #ifdef CONFIG_VALGRIND_H
diff --git a/.gitmodules b/.gitmodules
index 08b1b48a09..6434b98c49 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -64,3 +64,6 @@
 [submodule "roms/vbootrom"]
 	path = roms/vbootrom
 	url = https://gitlab.com/qemu-project/vbootrom.git
+[submodule "libucontext"]
+	path = subprojects/libucontext
+	url = https://github.com/kaniini/libucontext.git
diff --git a/MAINTAINERS b/MAINTAINERS
index f22d83c178..76de0c7dcb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2893,6 +2893,12 @@ F: hw/core/clock-vmstate.c
 F: hw/core/qdev-clock.c
 F: docs/devel/clocks.rst
 
+libucontext in QEMU
+M: Joelle van Dyne <j@getutm.app>
+S: Maintained
+F: subprojects/libucontext
+K: ^Subject:.*(?i)libucontext
+
 Usermode Emulation
 ------------------
 Overall usermode emulation
diff --git a/meson_options.txt b/meson_options.txt
index 9734019995..5db4ef21f8 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -108,6 +108,8 @@ option('fuse', type: 'feature', value: 'auto',
        description: 'FUSE block device export')
 option('fuse_lseek', type : 'feature', value : 'auto',
        description: 'SEEK_HOLE/SEEK_DATA support for FUSE exports')
+option('ucontext', type : 'feature', value : 'disabled',
+       description: 'libucontext support')
 
 option('vhost_user_blk_server', type: 'feature', value: 'auto',
        description: 'build vhost-user-blk server')
diff --git a/subprojects/libucontext b/subprojects/libucontext
new file mode 160000
index 0000000000..464f98a01b
--- /dev/null
+++ b/subprojects/libucontext
@@ -0,0 +1 @@
+Subproject commit 464f98a01b41006f9dc68ff0eee0aa2656709acc
-- 
2.28.0