Xen Static Shared Memory

[garethodickin]

Feature: Xen Static Shared Memory

Developer Contact Info:

taekyung.kang@boeing.com

Licensing

OSS Approved for Upstream

Feature request summary

* Add Xen static shared memory support on ARM64
* Shared memory regions declared in the device tree with compatible "xen,shared-memory-v1" are parsed at boot time and mapped into a dedicated virtual address range using pre-allocated L3 page tables
* A new API uk_shm_map() is provided to obtain a virtual address for a sub-range of a shared memory region and to control read/write permissions at page granularity
* Files added/modified:
  - plat/xen/Config.uk: Added XEN_STATIC_SHM, XEN_SHM_MAX_ENTRIES, XEN_SHM_REGION_SIZE_MB Kconfig options
  - plat/xen/Makefile.uk: Conditionally include shm.c when CONFIG_XEN_STATIC_SHM is enabled
  - plat/xen/arm/entry64.S: shm_l2_pgtable and shm_l3_pgtable allocation
  - plat/xen/arm/mm.c: Pool-based create_mapping() with L2/L3 page table management (alloc_init_pmd, alloc_init_pte)
  - plat/xen/arm/setup64.c: get_shm() for DTB parsing of "xen,shared-memory-v1" nodes and shm_pool initialization
  - plat/xen/arm/shm.c: uk_shm_map() implementation with spinlock protection, permission control, and TLB flush
  - plat/xen/include/xen-arm/shm.h: uk_shm_entry struct, uk_shm_map() API declaration, UK_SHM_RW/UK_SHM_RO defines
  - plat/xen/include/xen-arm/mm.h: SHM VA layout (L1[254], FIX_SHM_START/TOP), pgtable_pool struct, create_mapping() declaration

Describe alternatives

1. Xen grant tables: The existing Xen mechanism for inter-domain memory sharing. However, grant tables require runtime hypercalls for map/unmap operations. Additionally, grant tables depend on an active XenStore service and making them unsuitable for dom0less configurations. Static shared memory avoids this by declaring regions in the device tree and mapping them at boot time with zero runtime hypercall cost.

2. POSIX shared memory (shm_open/mmap): Only provides intra-domain (process-level) shared memory within a single OS instance. Not for inter-domain communication.

Related architectures

arm64

Related platforms

Xen

Additional context

* This feature implements the Xen static shared memory as defined in:
  https://xenbits.xen.org/docs/unstable/misc/arm/device-tree/booting.txt
* Device tree binding: compatible = "xen,shared-memory-v1", with "xen,id" property for region identification
* Virtual address layout: Uses a dedicated L1 page table entry (L1[254]) separate from the existing fixmap (L1[255])
* Configurable region size (default 64MB, range 2-1024MB, must be multiple of 2MB for L2 alignment)
* Maximum shared memory region entries (uk_shm_entry) configurable via Kconfig XEN_SHM_MAX_ENTRIES (default 32)