hw/i386/pc_sysfw: Alias rather than copy isa-bios region

In the -bios case the "isa-bios" memory region is an alias to the BIOS mapped
to the top of the 4G memory boundary. Do the same in the -pflash case, but only
for new machine versions for migration compatibility. This establishes common
behavior and makes pflash commands work in the "isa-bios" region which some
real-world legacy bioses rely on.

Note that in the sev_enabled() case, the "isa-bios" memory region in the -pflash
case will now also point to encrypted memory, just like it already does in the
-bios case.

When running `info mtree` before and after this commit with
`qemu-system-x86_64 -S -drive \
if=pflash,format=raw,readonly=on,file=/usr/share/qemu/bios-256k.bin` and running
`diff -u before.mtree after.mtree` results in the following changes in the
memory tree:

   --- before.mtree
   +++ after.mtree
   @@ -71,7 +71,7 @@
        0000000000000000-ffffffffffffffff (prio -1, i/o): pci
        00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
        00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
   -      00000000000e0000-00000000000fffff (prio 1, rom): isa-bios
   +      00000000000e0000-00000000000fffff (prio 1, romd): alias isa-bios @system.flash0 0000000000020000-000000000003ffff
        00000000000a0000-00000000000bffff (prio 1, i/o): alias smram-region @pci 00000000000a0000-00000000000bffff
        00000000000c0000-00000000000c3fff (prio 1, i/o): alias pam-pci @pci 00000000000c0000-00000000000c3fff
        00000000000c4000-00000000000c7fff (prio 1, i/o): alias pam-pci @pci 00000000000c4000-00000000000c7fff
   @@ -108,7 +108,7 @@
        0000000000000000-ffffffffffffffff (prio -1, i/o): pci
        00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
        00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
   -      00000000000e0000-00000000000fffff (prio 1, rom): isa-bios
   +      00000000000e0000-00000000000fffff (prio 1, romd): alias isa-bios @system.flash0 0000000000020000-000000000003ffff
        00000000000a0000-00000000000bffff (prio 1, i/o): alias smram-region @pci 00000000000a0000-00000000000bffff
        00000000000c0000-00000000000c3fff (prio 1, i/o): alias pam-pci @pci 00000000000c0000-00000000000c3fff
        00000000000c4000-00000000000c7fff (prio 1, i/o): alias pam-pci @pci 00000000000c4000-00000000000c7fff
   @@ -131,11 +131,14 @@
   memory-region: pc.ram
   0000000000000000-0000000007ffffff (prio 0, ram): pc.ram

   +memory-region: system.flash0
   +  00000000fffc0000-00000000ffffffff (prio 0, romd): system.flash0
   +
   memory-region: pci
   0000000000000000-ffffffffffffffff (prio -1, i/o): pci
        00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
        00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
   -    00000000000e0000-00000000000fffff (prio 1, rom): isa-bios
   +    00000000000e0000-00000000000fffff (prio 1, romd): alias isa-bios @system.flash0 0000000000020000-000000000003ffff

   memory-region: smram
        00000000000a0000-00000000000bffff (prio 0, ram): alias smram-low @pc.ram 00000000000a0000-00000000000bffff

Note that in both cases the "system" memory region contains the entry

  00000000fffc0000-00000000ffffffff (prio 0, romd): system.flash0

but the "system.flash0" memory region only appears standalone when "isa-bios" is
an alias.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-ID: <20240508175507.22270-7-shentey@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

5 files changed, 14 insertions, 1 deletions

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index bfb46e9b54..4a2d6f5a97 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1816,6 +1816,7 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
     pcmc->has_reserved_memory = true;
     pcmc->enforce_aligned_dimm = true;
     pcmc->enforce_amd_1tb_hole = true;
+    pcmc->isa_bios_alias = true;
     /* BIOS ACPI tables: 128K. Other BIOS datastructures: less than 4K reported
      * to be used at the moment, 32K should be enough for a while.  */
     pcmc->acpi_data_size = 0x20000 + 0x8000;
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 99efb3c45c..ebb51de380 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -526,12 +526,15 @@ DEFINE_I440FX_MACHINE(v9_1, "pc-i440fx-9.1", NULL,
 
 static void pc_i440fx_9_0_machine_options(MachineClass *m)
 {
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
+
     pc_i440fx_9_1_machine_options(m);
     m->alias = NULL;
     m->is_default = false;
 
     compat_props_add(m->compat_props, hw_compat_9_0, hw_compat_9_0_len);
     compat_props_add(m->compat_props, pc_compat_9_0, pc_compat_9_0_len);
+    pcmc->isa_bios_alias = false;
 }
 
 DEFINE_I440FX_MACHINE(v9_0, "pc-i440fx-9.0", NULL,
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index bb53a51ac1..bd7db4abac 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -378,10 +378,12 @@ DEFINE_Q35_MACHINE(v9_1, "pc-q35-9.1", NULL,
 
 static void pc_q35_9_0_machine_options(MachineClass *m)
 {
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     pc_q35_9_1_machine_options(m);
     m->alias = NULL;
     compat_props_add(m->compat_props, hw_compat_9_0, hw_compat_9_0_len);
     compat_props_add(m->compat_props, pc_compat_9_0, pc_compat_9_0_len);
+    pcmc->isa_bios_alias = false;
 }
 
 DEFINE_Q35_MACHINE(v9_0, "pc-q35-9.0", NULL,
diff --git a/hw/i386/pc_sysfw.c b/hw/i386/pc_sysfw.c
index 82d37cb376..ac88ad4eb9 100644
--- a/hw/i386/pc_sysfw.c
+++ b/hw/i386/pc_sysfw.c
@@ -135,6 +135,7 @@ static void pc_system_flash_map(PCMachineState *pcms,
                                 MemoryRegion *rom_memory)
 {
     X86MachineState *x86ms = X86_MACHINE(pcms);
+    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
     hwaddr total_size = 0;
     int i;
     BlockBackend *blk;
@@ -184,7 +185,12 @@ static void pc_system_flash_map(PCMachineState *pcms,
 
         if (i == 0) {
             flash_mem = pflash_cfi01_get_memory(system_flash);
-            pc_isa_bios_init(&x86ms->isa_bios, rom_memory, flash_mem);
+            if (pcmc->isa_bios_alias) {
+                x86_isa_bios_init(&x86ms->isa_bios, rom_memory, flash_mem,
+                                  true);
+            } else {
+                pc_isa_bios_init(&x86ms->isa_bios, rom_memory, flash_mem);
+            }
 
             /* Encrypt the pflash boot ROM */
             if (sev_enabled()) {
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index e52290916c..ad9c3d9ba8 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -119,6 +119,7 @@ struct PCMachineClass {
     bool enforce_aligned_dimm;
     bool broken_reserved_end;
     bool enforce_amd_1tb_hole;
+    bool isa_bios_alias;
 
     /* generate legacy CPU hotplug AML */
     bool legacy_cpu_hotplug;