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

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index a3f764d..19bf810 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/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
@@ -504,7 +498,6 @@ 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
@@ -512,7 +505,6 @@ trampoline_return:
        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)