zircon/kernel/phys/runtime/x86/start.S

// Copyright 2020 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/asm.h>
#include <lib/arch/ticks.h>
#include <lib/arch/x86/cpuid-asm.h>
#include <lib/arch/x86/msr.h>
#include <zircon/tls.h>

#include <phys/runtime/stack.h>

// This is the entry point from the boot loader or thereabouts.
// It receives one argument, in %rsi, usually a pointer (physical address).
//
// In a ZBI executable, this is where zbi_kernel_t::entry points and
// %rsi holds the address of the data ZBI.
.function _start, global
  // As early as possible collect the time stamp.
  sample_ticks

  // Disable asynchronous interrupts in case the boot loader left them on.
  cli

  // Zero the non-CS segment registers in case the bootloader left anything
  // there: DS, ES, and SS are vestigial; FS is unused in the kernel; GS will
  // be reset below.
  xor %eax, %eax
  mov %eax, %ds
  mov %eax, %es
  mov %eax, %fs
  mov %eax, %gs
  mov %eax, %ss

  // Clear any incoming stack pointer so it can't be used accidentally
  // before the proper stack is set up below.
  xor %esp, %esp

  // Clear frame pointer: at the root of the call stack.
  xor %ebp, %ebp

  // Save the timestamp since %rax must be clobbered below.
  mov %rax, %rbx

  // Clear bss.  Note this assumes it's aligned to 8, which is ensured
  // by the bss declaration below.
  lea _edata(%rip), %rdi
  lea _end(%rip), %rcx
  sub %rdi, %rcx
  shr $3, %rcx
  xor %eax, %eax
  cld  // Assume nothing.  ABI requires that DF be clear.
  rep stosq

  // Save first argument to PhysMain in a call-saved register.
  // The time stamp is already safe in %rbx.
  mov %rsi, %r12

  // Clear the IDT to zero address and zero limit, so any trap is sure to
  // get a triple-fault.  We've just cleared the .bss containing the stack,
  // so these bytes are known to be zero.
  lidt boot_stack(%rip)

  // Get all the cpuid data in place.
  // This only needs the basic stack to call and return.
  lea (boot_stack + BOOT_STACK_SIZE)(%rip), %rsp
  call InitializeBootCpuid

  // Set up the secondary stacks and the thread pointer area.
  lea boot_thread_pointer(%rip), %rax

  // The wrgsbase instruction is more efficient when it's supported.  But using
  // it requires both checking CPUID for its support and then enabling it in
  // %cr4.  There is no real need to bother with any of that in assembly since
  // one wrmsr at boot is not costly enough to bother avoiding.  The kernel can
  // check for and enable fsgsbase instructions easily and cleanly from C++
  // before it does anything that is performance-sensitive enough to warrant
  // preferring them over the MSRs.
  wrmsr64 MSR_IA32_GS_BASE

.Lstack_guard:
  // Use the best hardware randomness available for the stack canary value.
  testl $CPUID_EXTF_RDSEED, (gBootCpuidExtf + CPUID_EBX)(%rip)
  jnz .Lrdseed
  testl $CPUID_FEATURE_RDRAND, (gBootCpuidFeature + CPUID_ECX)(%rip)
  jnz .Lrdrand

  // The only "randomness" readily available is our own load address, so
  // swizzle that in with some arbitrary bits.
  lea _start(%rip), %rcx
  movabs $0xdeadbeef1ee2d00d, %rax
  xor %rcx, %rax
  jmp .Lstack_guard_write

.Lrdrand:
  rdrand %rax
  jmp .Lstack_guard_write

.Lrdseed:
  rdseed %rax

.Lstack_guard_write:
  mov %rax, %gs:ZX_TLS_STACK_GUARD_OFFSET

#if __has_feature(safe_stack)
  lea (boot_unsafe_stack + BOOT_STACK_SIZE)(%rip), %rax
  mov %rax, %gs:ZX_TLS_UNSAFE_SP_OFFSET
#endif

  // Now the full C++ ABI is available. Apply our own relocations before
  // anything that might depend on them. The PhysMain arguments are safe in
  // call-saved %r12 and %rbx.
  call ApplyRelocationsToSelf

  // This could theoretically be a tail call since it's obliged never to
  // return, but it's nice to have the caller in a backtrace (and the call
  // implicitly adjusts the stack alignment as the ABI requires).
  mov %r12, %rdi
  mov %rbx, %rsi
  call CppPhysMain

  // Trap forever just in case it does return.
0:ud2
  jmp 0b
.end_function

.object boot_thread_area, bss, local, align=8
#if ZX_TLS_UNSAFE_SP_OFFSET < ZX_TLS_STACK_GUARD_OFFSET
  .error "TLS ABI layout??"
#endif
.label boot_thread_pointer, global
  .skip ZX_TLS_UNSAFE_SP_OFFSET + 8
.end_object