[Leechael]

Problem

On usw5, eight B300 GPUs (10de:3182) expose 512 GiB + 64 MiB + 32 MiB of 64-bit MMIO BARs each: 4 TiB + 768 MiB in total. With qemu_pci_hole64_size = "4T", firmware expands the PCI64 window beyond the fixed window modeled by dstack-mr. KMS then rejects the ACPI digest after several minutes of boot, and repeated boots can interrupt GPU initialization/cleanup.

Fix

• Check the resolved passthrough GPUs and NVSwitch devices before GPU sanitization or QEMU launch. Read only sysfs BAR resource rows 0–5 and sum 64-bit MMIO sizes, excluding ROM and SR-IOV VF resources.
• Reject requirements larger than the configured PCI64 window (0 uses Q35's 32 GiB default). Return an actionable error, append the existing boot.error event and update boot_error, and clear the persisted started marker so automatic restart does not retry the same invalid configuration.
• Reuse the check in the one-shot launch path.

This is intentionally a small capacity check, not a firmware allocation simulator: alignment gaps can still require a larger window. No guest/KMS/protobuf/Cloud API changes. Existing CreateVm cleanup on synchronous start failure is unchanged; retained VMs started through StartVm expose the error through VmInfo.events as before.

Verification

• One compact inline regression test uses BAR rows captured from usw5 on 2026-10-30: eight B300s fail at 4 TiB and pass at 8 TiB. Also checks a derived 256 GiB BAR2 configuration, the default size, empty devices, and parsing/exclusion behavior.
• Executed the exact helper and inline test in a temporary standalone Cargo harness with the repository's locked anyhow/fs-err versions: 1 test passed. Production helper Clippy checks passed in that harness. Surrounding GPU config types were represented by minimal matching structs; this is not a full VMM build.
• Changed-file rustfmt checks (edition 2021) and git diff --check passed.
• Full VMM tests were not run locally: the host is macOS with no running Linux container daemon, and the offline workspace test command is blocked by missing dcap-qvl-webpki 0.103.15+dcap.1. Linux integration/build checks are left to CI.