[v1] hw/smbios: allow clearing the VM bit in SMBIOS table 0

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

Daniil Tatianin posted 1 patch 3 months, 3 weeks ago

Diff against v1 v2
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git fetch https://github.com/patchew-project/qemu tags/patchew/20250724194731.42701-1-d-tatianin@yandex-team.ru

Maintainers: "Michael S. Tsirkin" <mst@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Ani Sinha <anisinha@redhat.com>

There is a newer version of this series

hw/smbios/smbios.c           | 11 ++++++++++-
include/hw/firmware/smbios.h |  2 +-
2 files changed, 11 insertions(+), 2 deletions(-)

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

Posted by Daniil Tatianin 3 months, 3 weeks ago

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>
---

After talking to Michael:
Supersedes: <20250723090128.977364-1-d-tatianin@yandex-team.ru>

---
 hw/smbios/smbios.c           | 11 ++++++++++-
 include/hw/firmware/smbios.h |  2 +-
 2 files changed, 11 insertions(+), 2 deletions(-)

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/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;
-- 
2.34.1