pc: Kill the "use flash device for BIOS unless KVM" misfeature

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
"rom_only":

* 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 (and still doesn'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.

Fix as follows:

1. Change the meaning of rom_only=0 to mean "use flash, no ifs, buts,
or maybes" for pc-i440fx-1.5 and pc-q35-1.5.  Don't change anything
for older machines (to remain bug-compatible).

2. Change the default value from 0 to 1 for these machines.
Necessary, because 0 doesn't work with KVM.  Once it does, we can flip
the default back to 0.

3. Don't revert the retroactive addition of device "pc-sysfw" to older
machine types.  Seems not worth the trouble.

4. Add a TODO comment asking for device "pc-sysfw" to be dropped once
flash works with KVM.

Net effect is that you get a BIOS ROM again even when KVM is disabled,
just like for machines predating the introduction of flash.

To get flash instead, use "--global pc-sysfw.rom_only=0".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1365780303-26398-4-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 13e1d50fc0..89b4cb4635 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -235,9 +235,16 @@ static void pc_init_pci(QEMUMachineInitArgs *args)
              initrd_filename, cpu_model, 1, 1);
 }
 
+static void pc_init_pci_1_4(QEMUMachineInitArgs *args)
+{
+    pc_sysfw_flash_vs_rom_bug_compatible = true;
+    pc_init_pci(args);
+}
+
 static void pc_init_pci_1_3(QEMUMachineInitArgs *args)
 {
     enable_compat_apic_id_mode();
+    pc_sysfw_flash_vs_rom_bug_compatible = true;
     pc_init_pci(args);
 }
 
@@ -246,6 +253,7 @@ static void pc_init_pci_1_2(QEMUMachineInitArgs *args)
 {
     disable_kvm_pv_eoi();
     enable_compat_apic_id_mode();
+    pc_sysfw_flash_vs_rom_bug_compatible = true;
     pc_init_pci(args);
 }
 
@@ -318,7 +326,7 @@ static QEMUMachine pc_i440fx_machine_v1_5 = {
 static QEMUMachine pc_i440fx_machine_v1_4 = {
     .name = "pc-i440fx-1.4",
     .desc = "Standard PC (i440FX + PIIX, 1996)",
-    .init = pc_init_pci,
+    .init = pc_init_pci_1_4,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_1_4,