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From 62868855d0fdd34dee119a59c7e56c1706fc27ab Mon Sep 17 00:00:00 2001
From: Miroslav Rezanina <>
Date: Wed, 31 Jul 2013 08:00:55 +0200
Subject: pc: Disable the "use flash device for BIOS unless KVM" misfeature

Message-id: <>
Patchwork-id: 52417
O-Subject: [RHEL-7 PATCH qemu-kvm 1/1] pc: Disable the "use flash device for BIOS unless KVM" misfeature
Bugzilla: 963280
RH-Acked-by: Michael S. Tsirkin <>
RH-Acked-by: Laszlo Ersek <>
RH-Acked-by: Paolo Bonzini <>

Use of a flash memory device for the BIOS was added in series "[PATCH
v10 0/8] PC system flash support", commit 4732dca..1b89faf, v1.1.

Flash vs. ROM is a guest-visible difference.  Thus, flash use had to
be suppressed for machine types pc-1.0 and older.  This was
accomplished by adding a dummy device "pc-sysfw" with property

* Non-zero rom_only means "use ROM".  Default for pc-1.0 and older.
* Zero rom_only means "maybe use flash".  Default for newer machines.

Not only is the dummy device ugly, it was also retroactively added to
the older machine types!  Fortunately, it's not guest-visible (thus no
immediate guest ABI breakage), and has no vmstate (thus no immediate
migration breakage).  Breakage occurs only if the user unwisely
enables flash by setting rom_only to zero.  Patch review FAIL #1.

Why "maybe use flash"?  Flash didn't work with KVM.  Therefore,
rom_only=0 really means "use flash, except when KVM is enabled, use
ROM".  This is a Bad Idea, because it makes enabling/ disabling KVM
guest-visible.  Patch review FAIL #2.

Aside: it also precludes migrating between KVM on and off, but that's
not possible for other reasons anyway.

Upstream commit 9953f88 killed the misfeature for new machine types.
Upstream commit 9e1c2ec reverted the kill, but left a dead variable
behind.  This is what's in RHEL-7 is before this patch.

KVM has recently been made to support flash (KVM_CAP_READONLY_MEM),
which permitted upstream QEMU to redo the flash vs. ROM switch
properly (commit 338ea90..dafb82e).  The required follow-up cleanup
hasn't been committed, yet: Paolo's [PATCH v2 0/3] Remove legacy sysfw

Instead of backporting all that, I'm simply ripping out the flash
feature.  ROM has been working just fine for us, and I don't think we
should complicate RHEL-7 maintenance and support by having two ways to
map BIOS, at least not without sufficient business reason.

The result behaves just like upstream with the follow-up cleanup
applied, except -pflash doesn't affect the BIOS mapping, but remains
as it always was: it creates an unused drive.

Signed-off-by: Markus Armbruster <>

diff --git a/hw/block/pc_sysfw.c b/hw/block/pc_sysfw.c
index 4f17668..2bbedc9 100644
--- a/hw/block/pc_sysfw.c
+++ b/hw/block/pc_sysfw.c
@@ -36,6 +36,7 @@
 #define BIOS_FILENAME "bios.bin"
+#if 0   /* Disabled for Red Hat Enterprise Linux */
 typedef struct PcSysFwDevice {
     SysBusDevice busdev;
     uint8_t rom_only;
@@ -138,6 +139,7 @@ static void pc_system_flash_init(MemoryRegion *rom_memory,
     pc_isa_bios_init(rom_memory, flash_mem, size);
+#endif  /* Disabled for Red Hat Enterprise Linux */
 static void old_pc_system_rom_init(MemoryRegion *rom_memory)
@@ -194,6 +196,7 @@ static void old_pc_system_rom_init(MemoryRegion *rom_memory)
+#if 0   /* Disabled for Red Hat Enterprise Linux */
  * Bug-compatible flash vs. ROM selection enabled?
  * A few older machines enable this.
@@ -287,3 +290,9 @@ static void pcsysfw_register (void)
 type_init (pcsysfw_register);
+#else   /* Disabled for Red Hat Enterprise Linux */
+void pc_system_firmware_init(MemoryRegion *rom_memory)
+    old_pc_system_rom_init(rom_memory);