hw/smbios: allow clearing the VM bit in SMBIOS table 0

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>

diff --git a/hw/smbios/smbios.c b/hw/smbios/smbios.c
index 1ac063cfb4ba89d5a25844ae21a76bc524b9a5b2..13e21a9c43e9c42c3d68d92a3a369722da103c7b 100644 (file)
--- 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/include/hw/firmware/smbios.h b/include/hw/firmware/smbios.h
index f066ab726291238dc163a5ac729e8ef0144464ff..3ea732f4e6a6012fc8e2f0746798dc62a2ce198a 100644 (file)
--- 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/qemu-options.hx b/qemu-options.hx
index 075f4be2e3e67ee82f90bca6542a5adb30bb2f4d..3ff10ec09d9c6a78cb189d757235f5bd81c0f931 100644 (file)
--- 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"