[PULL 32/75] hw/smbios: allow clearing the VM bit in SMBIOS table 0

["Michael S. Tsirkin" <mst@redhat.com>]

From: Daniil Tatianin <d-tatianin@yandex-team.ru>

This is useful to be able to freeze a specific version of SeaBIOS to
prevent guest visible changes between BIOS updates. This is currently
not possible since the extension byte 2 provided by SeaBIOS does not
set the VM bit, whereas QEMU sets it unconditionally.

Allowing to clear it also seems useful if we want to hide the fact that
the guest system is running inside a virtual machine.

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20250724195409.43499-1-d-tatianin@yandex-team.ru>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/firmware/smbios.h |  2 +-
 hw/smbios/smbios.c           | 11 ++++++++++-
 qemu-options.hx              |  2 +-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/hw/firmware/smbios.h b/include/hw/firmware/smbios.h
index f066ab7262..3ea732f4e6 100644
--- a/include/hw/firmware/smbios.h
+++ b/include/hw/firmware/smbios.h
@@ -22,7 +22,7 @@ extern GArray *usr_blobs_sizes;
 
 typedef struct {
     const char *vendor, *version, *date;
-    bool have_major_minor, uefi;
+    bool have_major_minor, uefi, vm;
     uint8_t major, minor;
 } smbios_type0_t;
 extern smbios_type0_t smbios_type0;
diff --git a/hw/smbios/smbios.c b/hw/smbios/smbios.c
index 1ac063cfb4..13e21a9c43 100644
--- a/hw/smbios/smbios.c
+++ b/hw/smbios/smbios.c
@@ -179,6 +179,10 @@ static const QemuOptDesc qemu_smbios_type0_opts[] = {
         .name = "uefi",
         .type = QEMU_OPT_BOOL,
         .help = "uefi support",
+    },{
+        .name = "vm",
+        .type = QEMU_OPT_BOOL,
+        .help = "virtual machine",
     },
     { /* end of list */ }
 };
@@ -574,10 +578,14 @@ static void smbios_build_type_0_table(void)
 
     t->bios_characteristics = cpu_to_le64(0x08); /* Not supported */
     t->bios_characteristics_extension_bytes[0] = 0;
-    t->bios_characteristics_extension_bytes[1] = 0x14; /* TCD/SVVP | VM */
+
+    t->bios_characteristics_extension_bytes[1] = 0x04; /* TCD/SVVP */
     if (smbios_type0.uefi) {
         t->bios_characteristics_extension_bytes[1] |= 0x08; /* |= UEFI */
     }
+    if (smbios_type0.vm) {
+        t->bios_characteristics_extension_bytes[1] |= 0x10; /* |= VM */
+    }
 
     if (smbios_type0.have_major_minor) {
         t->system_bios_major_release = smbios_type0.major;
@@ -1405,6 +1413,7 @@ void smbios_entry_add(QemuOpts *opts, Error **errp)
             save_opt(&smbios_type0.version, opts, "version");
             save_opt(&smbios_type0.date, opts, "date");
             smbios_type0.uefi = qemu_opt_get_bool(opts, "uefi", false);
+            smbios_type0.vm = qemu_opt_get_bool(opts, "vm", true);
 
             val = qemu_opt_get(opts, "release");
             if (val) {
diff --git a/qemu-options.hx b/qemu-options.hx
index 075f4be2e3..3ff10ec09d 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2700,7 +2700,7 @@ DEF("smbios", HAS_ARG, QEMU_OPTION_smbios,
     "-smbios file=binary\n"
     "                load SMBIOS entry from binary file\n"
     "-smbios type=0[,vendor=str][,version=str][,date=str][,release=%d.%d]\n"
-    "              [,uefi=on|off]\n"
+    "              [,uefi=on|off][,vm=on|off]\n"
     "                specify SMBIOS type 0 fields\n"
     "-smbios type=1[,manufacturer=str][,product=str][,version=str][,serial=str]\n"
     "              [,uuid=uuid][,sku=str][,family=str]\n"
-- 
MST