x86/decompressor: Store boot_params pointer in callee save register

Instead of pushing and popping %RSI several times to preserve the struct
boot_params pointer across the execution of the startup code, move it
into a callee save register before the first call into C, and copy it
back when needed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-8-ardb@kernel.org

arch/x86/boot/compressed/head_64.S

@@ -405,10 +405,14 @@ SYM_CODE_START(startup_64)
  lretq
 
 .Lon_kernel_cs:
+ /*
+ * RSI holds a pointer to a boot_params structure provided by the
+ * loader, and this needs to be preserved across C function calls. So
+ * move it into a callee saved register.
+ */
+ movq %rsi, %r15
 
- pushq %rsi
  call load_stage1_idt
- popq %rsi
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
  /*
@@ -419,12 +423,10 @@ SYM_CODE_START(startup_64)
  * CPUID instructions being issued, so go ahead and do that now via
  * sev_enable(), which will also handle the rest of the SEV-related
  * detection/setup to ensure that has been done in advance of any dependent
- * code.
+ * code. Pass the boot_params pointer as the first argument.
  */
- pushq %rsi
- movq %rsi, %rdi /* real mode address */
+ movq %r15, %rdi
  call sev_enable
- popq %rsi
 #endif
 
  /*
@@ -437,13 +439,10 @@ SYM_CODE_START(startup_64)
  * - Non zero RDX means trampoline needs to enable 5-level
  * paging.
  *
- * RSI holds real mode data and needs to be preserved across
- * this function call.
+ * Pass the boot_params pointer as the first argument.
  */
- pushq %rsi
- movq %rsi, %rdi /* real mode address */
+ movq %r15, %rdi
  call paging_prepare
- popq %rsi
 
  /* Save the trampoline address in RCX */
  movq %rax, %rcx
@@ -456,9 +455,9 @@ SYM_CODE_START(startup_64)
  * because the architecture does not guarantee that GPRs will retain
  * their full 64-bit values across a 32-bit mode switch.
  */
+ pushq %r15
  pushq %rbp
  pushq %rbx
- pushq %rsi
 
  /*
  * Push the 64-bit address of trampoline_return() onto the new stack.
@@ -475,9 +474,9 @@ SYM_CODE_START(startup_64)
  lretq
 trampoline_return:
  /* Restore live 64-bit registers */
- popq %rsi
  popq %rbx
  popq %rbp
+ popq %r15
 
  /* Restore the stack, the 32-bit trampoline uses its own stack */
  leaq rva(boot_stack_end)(%rbx), %rsp
@@ -487,14 +486,9 @@ trampoline_return:
  *
  * RDI is address of the page table to use instead of page table
  * in trampoline memory (if required).
- *
- * RSI holds real mode data and needs to be preserved across
- * this function call.
  */
- pushq %rsi
  leaq rva(top_pgtable)(%rbx), %rdi
  call cleanup_trampoline
- popq %rsi
 
  /* Zero EFLAGS */
  pushq $0
@@ -504,15 +498,13 @@ trampoline_return:
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
  */
- pushq %rsi
  leaq (_bss-8)(%rip), %rsi
  leaq rva(_bss-8)(%rbx), %rdi
  movl $(_bss - startup_32), %ecx
  shrl $3, %ecx
  std
  rep movsq
  cld
- popq %rsi
 
  /*
  * The GDT may get overwritten either during the copy we just did or
@@ -544,30 +536,28 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
  shrq $3, %rcx
  rep stosq
 
- pushq %rsi
  call load_stage2_idt
 
  /* Pass boot_params to initialize_identity_maps() */
- movq (%rsp), %rdi
+ movq %r15, %rdi
  call initialize_identity_maps
- popq %rsi
 
 /*
  * Do the extraction, and jump to the new kernel..
  */
- pushq %rsi /* Save the real mode argument */
- movq %rsi, %rdi /* real mode address */
+ /* pass struct boot_params pointer */
+ movq %r15, %rdi
  leaq boot_heap(%rip), %rsi /* malloc area for uncompression */
  leaq input_data(%rip), %rdx /* input_data */
  movl input_len(%rip), %ecx /* input_len */
  movq %rbp, %r8 /* output target address */
  movl output_len(%rip), %r9d /* decompressed length, end of relocs */
  call extract_kernel /* returns kernel entry point in %rax */
- popq %rsi
 
 /*
  * Jump to the decompressed kernel.
  */
+ movq %r15, %rsi
  jmp *%rax
 SYM_FUNC_END(.Lrelocated)