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, 18 insertions, 2 deletions

diff --git a/hw/block/pc_sysfw.c b/hw/block/pc_sysfw.c
index 0d95c8a..aad8614 100644
--- a/hw/block/pc_sysfw.c
+++ b/hw/block/pc_sysfw.c
@@ -194,11 +194,23 @@ static void old_pc_system_rom_init(MemoryRegion *rom_memory)
                                 bios);
 }
 
+/*
+ * Bug-compatible flash vs. ROM selection enabled?
+ * A few older machines enable this.
+ */
+bool pc_sysfw_flash_vs_rom_bug_compatible;
+
 void pc_system_firmware_init(MemoryRegion *rom_memory)
 {
     DriveInfo *pflash_drv;
     PcSysFwDevice *sysfw_dev;
 
+    /*
+     * TODO This device exists only so that users can switch between
+     * use of flash and ROM for the BIOS.  The ability to switch was
+     * created because flash doesn't work with KVM.  Once it does, we
+     * should drop this device for new machine types.
+     */
     sysfw_dev = (PcSysFwDevice*) qdev_create(NULL, "pc-sysfw");
 
     qdev_init_nofail(DEVICE(sysfw_dev));
@@ -212,7 +224,11 @@ void pc_system_firmware_init(MemoryRegion *rom_memory)
 
     /* Currently KVM cannot execute from device memory.
        Use old rom based firmware initialization for KVM. */
-    if (kvm_enabled()) {
+    /*
+     * This is a Bad Idea, because it makes enabling/disabling KVM
+     * guest-visible.  Do it only in bug-compatibility mode.
+     */
+    if (pc_sysfw_flash_vs_rom_bug_compatible && kvm_enabled()) {
         if (pflash_drv != NULL) {
             fprintf(stderr, "qemu: pflash cannot be used with kvm enabled\n");
             exit(1);
@@ -239,7 +255,7 @@ void pc_system_firmware_init(MemoryRegion *rom_memory)
 }
 
 static Property pcsysfw_properties[] = {
-    DEFINE_PROP_UINT8("rom_only", PcSysFwDevice, rom_only, 0),
+    DEFINE_PROP_UINT8("rom_only", PcSysFwDevice, rom_only, 1),
     DEFINE_PROP_END_OF_LIST(),
 };