[RFC PATCH v4 4/6] xen: add option to disable legacy backends

[Volodymyr Babchuk]

This patch makes legacy backends optional. As was discussed at [1]
this is a solution to a problem when we can't run QEMU as a device
model in a non-privileged domain. This is because legacy backends
assume that they are always running in domain with ID = 0. Actually,
this may prevent running QEMU in a privileged domain with ID not equal
to zero.

With this patch it is possible to provide
"--disable-xen-legacy-backends" configure option to get QEMU binary
that can run in a driver domain. With price of not be able to use
legacy backends of course.

[1]
https://lists.gnu.org/archive/html/qemu-devel/2023-11/msg05022.html

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>

---

I am not sure if I made correct changes to build system, so this patch
is tagged as RFC.

Changes in v3:
 - New patch in v3
---
 hw/9pfs/meson.build           |  4 +++-
 hw/display/meson.build        |  4 +++-
 hw/i386/pc.c                  |  2 ++
 hw/usb/meson.build            |  5 ++++-
 hw/xen/meson.build            | 11 ++++++++---
 hw/xen/xen-hvm-common.c       |  2 ++
 hw/xenpv/xen_machine_pv.c     |  2 ++
 meson.build                   |  5 +++++
 meson_options.txt             |  2 ++
 scripts/meson-buildoptions.sh |  4 ++++
 10 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/hw/9pfs/meson.build b/hw/9pfs/meson.build
index 2944ea63c3..e8306ba8d2 100644
--- a/hw/9pfs/meson.build
+++ b/hw/9pfs/meson.build
@@ -15,7 +15,9 @@ fs_ss.add(files(
 ))
 fs_ss.add(when: 'CONFIG_LINUX', if_true: files('9p-util-linux.c'))
 fs_ss.add(when: 'CONFIG_DARWIN', if_true: files('9p-util-darwin.c'))
-fs_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xen-9p-backend.c'))
+if have_xen_legacy_backends
+  fs_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xen-9p-backend.c'))
+endif
 system_ss.add_all(when: 'CONFIG_FSDEV_9P', if_true: fs_ss)
 
 specific_ss.add(when: 'CONFIG_VIRTIO_9P', if_true: files('virtio-9p-device.c'))
diff --git a/hw/display/meson.build b/hw/display/meson.build
index 344dfe3d8c..18d657f6b3 100644
--- a/hw/display/meson.build
+++ b/hw/display/meson.build
@@ -14,7 +14,9 @@ system_ss.add(when: 'CONFIG_PL110', if_true: files('pl110.c'))
 system_ss.add(when: 'CONFIG_SII9022', if_true: files('sii9022.c'))
 system_ss.add(when: 'CONFIG_SSD0303', if_true: files('ssd0303.c'))
 system_ss.add(when: 'CONFIG_SSD0323', if_true: files('ssd0323.c'))
-system_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xenfb.c'))
+if have_xen_legacy_backends
+  system_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xenfb.c'))
+endif
 
 system_ss.add(when: 'CONFIG_VGA_PCI', if_true: files('vga-pci.c'))
 system_ss.add(when: 'CONFIG_VGA_ISA', if_true: files('vga-isa.c'))
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 29b9964733..91857af428 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1263,7 +1263,9 @@ void pc_basic_device_init(struct PCMachineState *pcms,
             pci_create_simple(pcms->bus, -1, "xen-platform");
         }
         pcms->xenbus = xen_bus_init();
+#ifdef CONFIG_XEN_LEGACY_BACKENDS
         xen_be_init();
+#endif
     }
 #endif
 
diff --git a/hw/usb/meson.build b/hw/usb/meson.build
index e94149ebde..8d395745b2 100644
--- a/hw/usb/meson.build
+++ b/hw/usb/meson.build
@@ -84,6 +84,9 @@ if libusb.found()
   hw_usb_modules += {'host': usbhost_ss}
 endif
 
-system_ss.add(when: ['CONFIG_USB', 'CONFIG_XEN_BUS', libusb], if_true: 
files('xen-usb.c'))
+if have_xen_legacy_backends
+  system_ss.add(when: ['CONFIG_USB', 'CONFIG_XEN_BUS', libusb],
+                if_true: files('xen-usb.c'))
+endif
 
 modules += { 'hw-usb': hw_usb_modules }
diff --git a/hw/xen/meson.build b/hw/xen/meson.build
index d887fa9ba4..964c3364f2 100644
--- a/hw/xen/meson.build
+++ b/hw/xen/meson.build
@@ -2,11 +2,16 @@ system_ss.add(when: ['CONFIG_XEN_BUS'], if_true: files(
   'xen-backend.c',
   'xen-bus-helper.c',
   'xen-bus.c',
-  'xen-legacy-backend.c',
-  'xen_devconfig.c',
-  'xen_pvdev.c',
 ))
 
+if have_xen_legacy_backends
+  system_ss.add(when: ['CONFIG_XEN_BUS'], if_true: files(
+    'xen_pvdev.c',
+    'xen-legacy-backend.c',
+    'xen_devconfig.c',
+  ))
+endif
+
 system_ss.add(when: ['CONFIG_XEN', xen], if_true: files(
   'xen-operations.c',
 ))
diff --git a/hw/xen/xen-hvm-common.c b/hw/xen/xen-hvm-common.c
index 565dc39c8f..2e7897dbd2 100644
--- a/hw/xen/xen-hvm-common.c
+++ b/hw/xen/xen-hvm-common.c
@@ -869,7 +869,9 @@ void xen_register_ioreq(XenIOState *state, unsigned int 
max_cpus,
 
     xen_bus_init();
 
+#ifdef CONFIG_XEN_LEGACY_BACKENDS
     xen_be_init();
+#endif
 
     return;
 
diff --git a/hw/xenpv/xen_machine_pv.c b/hw/xenpv/xen_machine_pv.c
index 9f9f137f99..03a55f345c 100644
--- a/hw/xenpv/xen_machine_pv.c
+++ b/hw/xenpv/xen_machine_pv.c
@@ -37,7 +37,9 @@ static void xen_init_pv(MachineState *machine)
     setup_xen_backend_ops();
 
     /* Initialize backend core & drivers */
+#ifdef CONFIG_XEN_LEGACY_BACKENDS
     xen_be_init();
+#endif
 
     switch (xen_mode) {
     case XEN_ATTACH:
diff --git a/meson.build b/meson.build
index ec01f8b138..c8a43dd97d 100644
--- a/meson.build
+++ b/meson.build
@@ -1749,6 +1749,9 @@ have_xen_pci_passthrough = 
get_option('xen_pci_passthrough') \
            error_message: 'Xen PCI passthrough not available on this 
platform') \
   .allowed()
 
+have_xen_legacy_backends = 
get_option('xen-legacy-backends').require(xen.found(),
+           error_message: 'Xen legacy backends requested but Xen not 
enabled').allowed()
+
 
 cacard = not_found
 if not get_option('smartcard').auto() or have_system
@@ -2219,6 +2222,7 @@ config_host_data.set('CONFIG_DBUS_DISPLAY', dbus_display)
 config_host_data.set('CONFIG_CFI', get_option('cfi'))
 config_host_data.set('CONFIG_SELINUX', selinux.found())
 config_host_data.set('CONFIG_XEN_BACKEND', xen.found())
+config_host_data.set('CONFIG_XEN_LEGACY_BACKENDS', have_xen_legacy_backends)
 config_host_data.set('CONFIG_LIBDW', libdw.found())
 if xen.found()
   # protect from xen.version() having less than three components
@@ -3049,6 +3053,7 @@ config_all += config_targetos
 config_all += config_all_disas
 config_all += {
   'CONFIG_XEN': xen.found(),
+  'CONFIG_XEN_LEGACY_BACKENDS': have_xen_legacy_backends,
   'CONFIG_SYSTEM_ONLY': have_system,
   'CONFIG_USER_ONLY': have_user,
   'CONFIG_ALL': true,
diff --git a/meson_options.txt b/meson_options.txt
index c9baeda639..91dd677257 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -77,6 +77,8 @@ option('nvmm', type: 'feature', value: 'auto',
        description: 'NVMM acceleration support')
 option('xen', type: 'feature', value: 'auto',
        description: 'Xen backend support')
+option('xen-legacy-backends', type: 'feature', value: 'auto',
+       description: 'Xen legacy backends (9pfs, fb, qusb) support')
 option('xen_pci_passthrough', type: 'feature', value: 'auto',
        description: 'Xen PCI passthrough support')
 option('tcg', type: 'feature', value: 'enabled',
diff --git a/scripts/meson-buildoptions.sh b/scripts/meson-buildoptions.sh
index 680fa3f581..b5acef008f 100644
--- a/scripts/meson-buildoptions.sh
+++ b/scripts/meson-buildoptions.sh
@@ -218,6 +218,8 @@ meson_options_help() {
   printf "%s\n" '  werror          Treat warnings as errors'
   printf "%s\n" '  whpx            WHPX acceleration support'
   printf "%s\n" '  xen             Xen backend support'
+  printf "%s\n" '  xen-legacy-backends'
+  printf "%s\n" '                  Xen legacy backends (9pfs, fb, qusb) 
support'
   printf "%s\n" '  xen-pci-passthrough'
   printf "%s\n" '                  Xen PCI passthrough support'
   printf "%s\n" '  xkbcommon       xkbcommon support'
@@ -556,6 +558,8 @@ _meson_option_parse() {
     --disable-whpx) printf "%s" -Dwhpx=disabled ;;
     --enable-xen) printf "%s" -Dxen=enabled ;;
     --disable-xen) printf "%s" -Dxen=disabled ;;
+    --enable-xen-legacy-backends) printf "%s" -Dxen-legacy-backends=enabled ;;
+    --disable-xen-legacy-backends) printf "%s" -Dxen-legacy-backends=disabled 
;;
     --enable-xen-pci-passthrough) printf "%s" -Dxen_pci_passthrough=enabled ;;
     --disable-xen-pci-passthrough) printf "%s" -Dxen_pci_passthrough=disabled 
;;
     --enable-xkbcommon) printf "%s" -Dxkbcommon=enabled ;;
-- 
2.42.0