Expand ARMv8 MMIO32 for 256 MiB VMSVGA VRAM

[moreaki]

Summary

Expand the Armv8 PCI 32-bit MMIO aperture from 256 MiB to 448 MiB.

The source change is one value in the Armv8 machine configuration. The accompanying comment explains the alignment/layout constraint.

Why

A VMSVGA guest configured with 256 MiB VRAM needs a 256 MiB prefetchable VRAM BAR as well as the VMSVGA device MMIO BAR in low PCI address space. The existing 256 MiB Armv8 aperture cannot contain both.

A 448 MiB request is placed by the resource manager at the required 256 MiB-aligned 0xe0000000 base and ends at 0xfbffffff, below the platform devices already reserved at the top of the low MMIO hole.

Minimal reproduction

Host/guest configuration:

• macOS Arm64 host
• Windows 11 Arm guest
• VMSVGA with 3D acceleration enabled
• --vram 256

After starting the current patched build, inspect VBox.log:

rg "pci-mmio32|VRamSize|VMSVGA3dEnabled|VMSVGAEnabled" "$HOME/VirtualBox VMs/Win11/Logs/VBox.log"

Current patched result:

00000000e0000000 - 00000000fbffffff (469762048) MMIO pci-mmio32
VMSVGA3dEnabled       <integer> = 0x0000000000000001 (1)
VMSVGAEnabled         <integer> = 0x0000000000000001 (1)
VRamSize              <integer> = 0x0000000010000000 (268435456, 256.0 MiB)

The same guest reaches Guest Additions run level 3 and a 5760x3240x32 mode on the current integrated build.

Validation status

• Rebased independently on origin/main at 5ac81eda707fcf3fcb49f9281401ceac0fc70f4a.
• One source file, one functional value change; git diff --check passes.
• Included in a clean Darwin/Arm full build and a strictly signed staged app.
• Patched runtime log confirms the 448 MiB aperture, 256 MiB VRAM, VMSVGA, and 3D configuration shown above.
• The negative runtime A/B with the aperture change removed is still required before this should be merged. Exact failure/log output from that run will be added rather than inferred.

Refs #742.

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