[2/4] mm/pgalloc, arch/xtensa: Support BUILD_KERNEL on Xtensa - #19796
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The goal is #19798, together with the ESP32-S3 chip code and the user mode isolation that a kernel build needs. But we have split it. Please review this one. |
Add CONFIG_MM_PGSIZE == 32768 and 65536 to the page-size switch (and the Kconfig help text). The 64 KB size matches the ESP32-S3 cache-MMU page granularity, so an address-environment port there can use one mm_pgalloc() page per cache-MMU page (naturally 64 KB-aligned by the granule allocator) instead of coalescing several smaller pages. Inert for existing configs: MM_PGSIZE is only used when CONFIG_MM_PGALLOC is enabled (BUILD_KERNEL). Assisted-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
Add what a kernel build needs on Xtensa: a crt0 for a user process, the kernel stack allocation that a system call switches to, the syscall entry and return path for an unprivileged caller, and the initial register state that starts a user task at EL0 with its save area on the kernel stack. On the ESP32-S3 the arch code that runs while the flash mapping is in flux moves to IRAM, and the kernel heap is placed above the user .bss so that up_allocate_kheap() and the user address environment do not overlap. Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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Summary
Part 2 of 4, split from #19772. Two independent pieces, no chip or board code.
mm/pgallocgains 32 KB and 64 KB pages. It had 1 KB, 2 KB and 4 KB. The Espressif cache MMU has a fixed 64 KB page for external flash and 32 KB for external RAM.arch/xtensa/src/commongains whatBUILD_KERNELneeds and Xtensa lacked: a kernel stack for a user thread, a signal trampoline that returns through a system call, an initial register state that starts a user task at the right privilege, and the system call paths between the two worlds.One detail worth review. A stack pointer carries a base save area in the 16 bytes below it, holding the caller's spilled
a0-a3. Anything that builds or moves a stack pointer must leave it alone, so the signal trampoline reserves it explicitly rather than assuming the space below the pointer is free.This adds no
fork()orvfork()code and depends on nothing.Impact
None on any existing configuration. Xtensa can be built with
CONFIG_BUILD_KERNELafter this, but no chip provides an address environment yet, so nothing can be.mm/pgallocchanges no default.Testing
No kernel build is possible on this branch, so what is shown is that the common code compiles and does not disturb a flat build.
esp32s3-devkit:nshbuilds clean on this branch alone.esp32s3-devkit:ostestruns to the end on an ESP32-S3-DevKitC, WROOM-2 N32R8V:tools/checkpatch.sh -c -u -m -gpasses.