[Qemu-devel] [PATCH 3/3] pc: Kill the "use flash device for BIOS unless KVM" misfeature

[Markus Armbruster <armbru@redhat.com>]

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>
---
 hw/block/pc_sysfw.c  | 20 ++++++++++++++++++--
 hw/i386/pc_piix.c    | 10 +++++++++-
 hw/i386/pc_q35.c     |  8 +++++++-
 include/hw/i386/pc.h |  5 +++++
 4 files changed, 39 insertions(+), 4 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(),
 };
 
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index f2d3d27..9b01a0e 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -234,9 +234,16 @@ 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);
 }
 
@@ -245,6 +252,7 @@ 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);
 }
 
@@ -317,7 +325,7 @@ 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,
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 6ac1a89..8f0c3f7 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -209,6 +209,12 @@ static void pc_q35_init(QEMUMachineInitArgs *args)
     }
 }
 
+static void pc_q35_init_1_4(QEMUMachineInitArgs *args)
+{
+    pc_sysfw_flash_vs_rom_bug_compatible = true;
+    pc_q35_init(args);
+}
+
 static QEMUMachine pc_q35_machine_v1_5 = {
     .name = "pc-q35-1.5",
     .alias = "q35",
@@ -221,7 +227,7 @@ static QEMUMachine pc_q35_machine_v1_5 = {
 static QEMUMachine pc_q35_machine_v1_4 = {
     .name = "pc-q35-1.4",
     .desc = "Standard PC (Q35 + ICH9, 2009)",
-    .init = pc_q35_init,
+    .init = pc_q35_init_1_4,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_1_4,
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 5d40914..cf8819d 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -176,6 +176,7 @@ static inline bool isa_ne2000_init(ISABus *bus, int base, int irq, NICInfo *nd)
 }
 
 /* pc_sysfw.c */
+extern bool pc_sysfw_flash_vs_rom_bug_compatible;
 void pc_system_firmware_init(MemoryRegion *rom_memory);
 
 /* e820 types */
@@ -241,6 +242,10 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
             .driver   = "virtio-net-pci",\
             .property = "romfile",\
             .value    = "pxe-virtio.rom",\
+        },{\
+            .driver   = "pc-sysfw",\
+            .property = "rom_only",\
+            .value    = stringify(0),\
         }
 
 #endif
-- 
1.7.11.7