ArmVirtPkg/QemuVirtMemInfoLib: trim the MMIO region mapping

ArmVirtPkg/QemuVirtMemInfoLib: trim the MMIO region mapping

QEMU/mach-virt is rather unhelpful when it comes to tracking down
NULL pointer dereferences that occur while running in UEFI: since
we have NOR flash mapped at address 0x0, inadvertent reads go
unnoticed, and even most writes are silently dropped, unless you're
unlucky and the instruction in question is one that KVM cannot
emulate, in which case you end up with a QEMU crash like this:

  error: kvm run failed Function not implemented
   PC=000000013f7ff804 X00=000000013f7ab108 X01=0000000000000064
  X02=000000013f801988 X03=00000000800003c4 X04=0000000000000000
  X05=0000000096000044 X06=fffffffffffd8270 X07=000000013f7ab4a0
  X08=0000000000000001 X09=000000013f803b88 X10=000000013f7e88d0
  X11=0000000000000009 X12=000000013f7ab554 X13=0000000000000008
  X14=0000000000000002 X15=0000000000000000 X16=0000000000000000
  X17=0000000000000000 X18=0000000000000000 X19=0000000000000000
  X20=000000013f81c000 X21=000000013f7ab170 X22=000000013f81c000
  X23=0000000009000018 X24=000000013f407020 X25=000000013f81c000
  X26=000000013f803530 X27=000000013f802000 X28=000000013f7ab270
  X29=000000013f7ab0d0 X30=000000013f7fee10  SP=000000013f7a6f30
  PSTATE=800003c5 N--- EL1h

and a warning in the host kernel log that load/store instruction
decoding is not supported by KVM.

Given that the first page of the flash device is not actually
used anyway, let's reduce the mappings of the peripheral space
and the flash device (both of which cover page #0) to only cover
what is actually required:

  ArmVirtQemu.fdf:
  > 0x00001000|0x001ff000
  > gArmTokenSpaceGuid.PcdFvBaseAddress|gArmTokenSpaceGuid.PcdFvSize

  ArmVirtQemuKernel.fdf:
  > 0x00008000|0x001f8000
  > gArmTokenSpaceGuid.PcdFvBaseAddress|gArmTokenSpaceGuid.PcdFvSize

For ArmVirtQemu, the resulting virtual mapping looks roughly like:
- [0, 4K)       : flash, unmapped
- [4K, 2M)      : flash, mapped as WB+X RAM
- [2M, 64M)     : flash, unmapped
- [64M, 128M)   : varstore flash, will be mapped by the NOR flash driver
- [128M, 256M)  : peripherals, mapped as device
- [256M, 1GB)   : 32-bit MMIO aperture, translated IO aperture, ECAM,
                  will be mapped by the PCI host bridge driver
- [1GB, ...)    : RAM, mapped.

After this change, any inadvertent read or write from/to the first
physical page will trigger a translation fault inside the guest,
regardless of the nature of the instruction, without crashing QEMU.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>