zircon/kernel/phys/trampoline-boot-arm64-zbi-boot.cc

// Copyright 2025 The Fuchsia Authors
//
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT

#include <lib/arch/cache.h>
#include <stdint.h>

#include <phys/boot-zbi.h>

// TODO(https://fxbug.dev/408020980): When trampoline booting a kernel and ZBI
// that overlap with the current load image, it is difficult to guarantee that
// we do not accidentally generate dirty cache lines corresponding that next
// loaded images. That is, unless we do a full (set/way) local cache flush,
// which is slated for removal. As a switchback to removing this type of
// flushing elsewhere, we introduce a trampoline-boot-specific override to
// ZbiBoot() that still employs it. Eventually we'll remove the ability to load
// images that overlap with the current load image (which is a feature only
// exercised in tests) and remove this override.

void BootZbi::ZbiBoot(uintptr_t entry, void* data) const {
  arch::DisableLocalCachesAndMmu();
  // Clear the stack and frame pointers and the link register so no misleading
  // breadcrumbs are left.
  __asm__ volatile(
      R"""(
      mov x0, %[zbi]
      mov x29, xzr
      mov x30, xzr
      mov sp, x29
      br %[entry]
      )"""
      :
      : [entry] "r"(entry), [zbi] "r"(data)
      // The compiler gets unhappy if x29 (fp) is a clobber.  It's never going
      // to be the register used for %[entry] anyway.  The memory clobber is
      // probably unnecessary, but it expresses that this constitutes access to
      // the memory kernel and zbi point to.
      : "x0", "x30", "memory");
  __builtin_unreachable();
}