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

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>