microvm with ACPI enabled places initrd over ACPI tables when RAM is 2048M
## Host environment
- Operating system: Arch Linux
- OS/kernel version: Linux lightquantum-arch 7.0.1-1-cachyos #1 SMP PREEMPT Thu, 23 Apr 2026 21:04:50 +0000 x86_64 GNU/Linux
- Architecture: x86
- QEMU flavor: qemu-system-x86_64
- QEMU version: QEMU emulator version 11.0.0
- QEMU command line:
```
qemu-system-x86_64 \
-M microvm,acpi=on \
-m 2048 \
-kernel ./vmlinuz-lts \
-initrd ./initramfs-lts \
-append 'console=ttyS0 loglevel=7' \
-serial stdio \
-display none
```
## Emulated/Virtualized environment
- Operating system: Alpine Linux 15.2.0 (netboot initramfs)
- OS/kernel version: Linux version 6.18.22-0-lts (buildozer@build-3-23-x86_64) (cc (Alpine 15.2.0) 15.2.0, GNU ld (GNU Binutils) 2.45.1) #1-Alpine SMP PREEMPT_DYNAMIC 2026-04-11 15:53:25
- Architecture: x86
## Description of problem
When booting Linux initrd with `-M microvm,acpi=on`, and when the memory size is exactly 2GiB, QEMU places the initrd at 0x7fffffff. The ACPI tables are also allocated in that same region, so the initrd and ACPI tables can overlap. This
sometimes corrupts the ACPI tables.
Here's the relevant log:
```
[ 0.000000] Linux version 6.18.22-0-lts (buildozer@build-3-23-x86_64) (cc (Alpine 15.2.0) 15.2.0, GNU ld (GNU Binutils) 2.45.1) #1-Alpine SMP PREEMPT_DYNAMIC 2026-04-11 15:53:25
[ 0.000000] Command line: console=ttyS0 loglevel=7
[ 0.000000] BIOS-provided physical RAM map:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007fffefff] usable
[ 0.000000] BIOS-e820: [mem 0x000000007ffff000-0x000000007fffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
...
[ 0.047398] RAMDISK: [mem 0x7e656000-0x7fffffff]
[ 0.047552] Allocated new RAMDISK: [mem 0x7ccac000-0x7e655e4a]
[ 0.112916] Move RAMDISK from [mem 0x7e656000-0x7ffffe4a] to [mem 0x7ccac000-0x7e655e4a]
[ 0.113513] ACPI: Early table checksum verification disabled
[ 0.113995] ACPI: RSDP 0x00000000000F7A70 000024 (v02 BOCHS )
[ 0.114326] ACPI: XSDT 0x000000007FFFFFAE 000034 (v01 BOCHS BXPC 00000001 BXPC 00000001)
[ 0.115021] ACPI: FACP 0x000000007FFFFE50 00010C (v05 BOCHS BXPC 00000001 BXPC 00000001)
[ 0.115564] ACPI: 0x000000007FFFFD00 000000 (v00 00000000 00000000)
[ 0.115698] ACPI: APIC 0x000000007FFFFF5C 000052 (v03 BOCHS BXPC 00000001 BXPC 00000001)
[ 0.115847] ACPI: Reserving FACP table memory at [mem 0x7ffffe50-0x7fffff5b]
[ 0.119285] No NUMA configuration found
...
[ 0.568685] ACPI Error: AE_NO_ACPI_TABLES, While loading namespace from ACPI tables (20250807/tbxfload-68)
```
You can see that the RAM disk address space `0x7e656000-0x7fffffff` (actual range `0x7e656000-0x7ffffe4a`) and the ACPI tables space `0x7ffffd00-` overlap. In this specific run, the DSDT table is corrupted.
Suspected cause: https://gitlab.com/qemu-project/qemu/-/blob/v11.0.0/hw/i386/microvm.c?ref_type=tags#L333 passes `acpi_data_size=0`. We should probably set it to be the size of the ACPI table when acpi=on.
[The document](https://www.qemu.org/docs/master/system/i386/microvm.html) says "(microvm) is a minimalist machine type without PCI nor ACPI support," but by
https://gitlab.com/qemu-project/qemu/-/commit/8045df14bccaeec39291c3d83fea2f638656b5d0, I assume microvm with ACPI is supported.
Downstream bug: https://github.com/microvm-nix/microvm.nix/issues/171
## Steps to reproduce
1. Download `https://dl-cdn.alpinelinux.org/alpine/v3.23/releases/x86_64/netboot/vmlinuz-lts` and `https://dl-cdn.alpinelinux.org/alpine/v3.23/releases/x86_64/netboot/initramfs-lts`
2. Pad the initrd a bit by `dd if=/dev/zero bs=1 count=1024 >> initramfs-lts`. This is to make sure it overwrites and corrupts the ACPI tables. Note that the original Alpine initrd is of a "lucky" size (initrd_size 0x19a9a4b mod 4096 = 0xa4b < 0xd00) and avoids the table. After padding, it's (0x19a9e4b mod 4096 = 0xe4b > 0xd00).
3. Boot QEMU VM and inspect the log
## Additional information
[log](https://pastebin.com/1CumihsL)
issue
GitLab AI Context
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