openvmm_entry,vmm_core: unblock and test pcie with x64 linux direct

[jackschefer-msft]

This change:

• Moves ECAM below 4G to appease the linux kernel
• Removes the legacy PCI bus from unenlightened linux direct
• Generates ACPI tables (an SSDT) describing PCIe roots for x64 linux direct like we do for UEFI

[Copilot]

Pull request overview

This PR enables PCIe support for x64 Linux direct boot in OpenVMM by addressing three issues that prevented it from working: the ECAM was placed above 4GB (at 32GB) which the Linux kernel doesn't support, the legacy PCI bus conflicted with PCIe, and ACPI tables describing PCIe roots (SSDT) were not generated for the Linux direct path.

Changes:

• Moves the PCIe ECAM region below 4GB by placing it just below the low PCI MMIO range (which is itself below the platform MMIO gap), instead of at a fixed 32GB base address. Removes the now-unused DEFAULT_PCIE_ECAM_BASE constant.
• Removes the legacy PCI bus from unenlightened Linux direct boot (with_generic_pci_bus: false) to avoid conflicts with PCIe.
• Generates ACPI SSDT tables describing PCIe root complexes in the common build_acpi_tables_inner path (used by Linux direct), matching what was already done for UEFI. Adds a linux_direct_x64 test variant and updates PCI device parsing to use sysfs and full 24-bit class codes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
openvmm/openvmm_defs/src/config.rs	Removes the DEFAULT_PCIE_ECAM_BASE constant (was 32GB, above 4G)
openvmm/openvmm_entry/src/lib.rs	Restructures ECAM placement to be below low PCI MMIO (below 4G), removes DEFAULT_PCIE_ECAM_BASE import
vmm_core/vm_manifest_builder/src/lib.rs	Sets with_generic_pci_bus: false for UnenlightenedLinuxDirect; updates doc comment
vmm_core/src/acpi_builder.rs	Adds SSDT generation for PCIe host bridges in build_acpi_tables_inner, used by Linux direct boot
vmm_tests/vmm_tests/tests/tests/multiarch/pcie.rs	Adds linux_direct_x64 test variant, switches Linux PCI enumeration from lspci to sysfs, changes class_code from u16 to u32 (full 24-bit class code)

[github-actions]

At least one Petri test failed.