Module layout

Constants§

GUEST_VA_BASE_DEFAULT: Base VA at which the guest’s entire memory range is mapped. Both the host (via mmap of snapshot/scratch regions) and the guest (via its page table) use this as the anchor. Configurable via JAR_GUEST_VA_BASE env var (hex string, with or without 0x prefix); default chosen to sit in the practically-never-touched mid-range band of x86_64 user VA space.
GUEST_VA_SIZE: Total VA range reserved for the guest. Layout inside: [0, 4 GiB) javm program; [4, 5 GiB) JIT scratch; [5 GiB, 7 GiB) kernel (KERNEL_OFFSET); [7 GiB, end) scratch.
KERNEL_OFFSET: Offset within the reservation where the kernel binary loads.
MAX_GPA: We assume 36-bit IPAs for now, since every amd64 processor supports at least 36 bits. Almost all of them support at least 40 bits, so we could consider bumping this in the future if we were ever memory-constrained.
MAX_GVA: We have this the top of the page below the top of memory in order to make working with start/end ptrs in a few places more convenient (not needing to worry about overflow)
SCRATCH_TOP_ALLOCATOR_OFFSET
SCRATCH_TOP_EXN_STACK_OFFSET
SCRATCH_TOP_SIZE_OFFSET
SCRATCH_TOP_SNAPSHOT_GENERATION_OFFSET
SCRATCH_TOP_SNAPSHOT_PT_GPA_BASE_OFFSET
SNAPSHOT_PT_GVA_MAX
SNAPSHOT_PT_GVA_MIN

guest_va_base
min_scratch_size: Compute the minimum scratch region size needed for a sandbox. On amd64, this is:
reserve_guest_va_range: One-time process-wide reservation of the [guest_va_base(), guest_va_base() + GUEST_VA_SIZE) range. Done on host startup so later mmaps of guest-visible regions (snapshot, scratch, kernel shadow) can land at known fixed VAs via MAP_FIXED inside this reservation.
scratch_base_gpa
scratch_base_gva