x86: fix early boot output

Loading the VGA base address involves sym_esi(), i.e. %esi still needs
to hold the relocation base address. Therefore the address of the
message to output cannot be "passed" in %esi. Put the message offset in
%ecx instead, adding it into %esi _after_ its last use as base address.

Fixes: b28044226e1c ("x86: make Xen early boot code relocatable")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: b1c16800e52743d9afd9af62c810f03af16dd942
master date: 2023-07-19 10:22:56 +0200

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 0fb7dd3029f2b6160a7e33796fb336ef43f413f2..245c859dd7b8db39fd6051b0a24c49d0b891972b 100644 (file)
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -141,21 +141,21 @@ efi_platform:
         .section .init.text, "ax", @progbits
 
 bad_cpu:
-        add     $sym_offs(.Lbad_cpu_msg),%esi   # Error message
+        mov     $sym_offs(.Lbad_cpu_msg), %ecx
         jmp     .Lget_vtb
 not_multiboot:
-        add     $sym_offs(.Lbad_ldr_msg),%esi   # Error message
+        mov     $sym_offs(.Lbad_ldr_msg), %ecx
         jmp     .Lget_vtb
 .Lmb2_no_st:
         /*
          * Here we are on EFI platform. vga_text_buffer was zapped earlier
          * because there is pretty good chance that VGA is unavailable.
          */
-        add     $sym_offs(.Lbad_ldr_nst),%esi   # Error message
+        mov     $sym_offs(.Lbad_ldr_nst), %ecx
         jmp     .Lget_vtb
 .Lmb2_no_ih:
         /* Ditto. */
-        add     $sym_offs(.Lbad_ldr_nih),%esi   # Error message
+        mov     $sym_offs(.Lbad_ldr_nih), %ecx
         jmp     .Lget_vtb
 .Lmb2_no_bs:
         /*
@@ -163,7 +163,7 @@ not_multiboot:
          * via start label. Then reliable vga_text_buffer zap is impossible
          * in Multiboot2 scanning loop and we have to zero %edi below.
          */
-        add     $sym_offs(.Lbad_ldr_nbs),%esi   # Error message
+        mov     $sym_offs(.Lbad_ldr_nbs), %ecx
         xor     %edi,%edi                       # No VGA text buffer
         jmp     .Lprint_err
 .Lmb2_efi_ia_32:
@@ -171,12 +171,15 @@ not_multiboot:
          * Here we are on EFI IA-32 platform. Then reliable vga_text_buffer zap is
          * impossible in Multiboot2 scanning loop and we have to zero %edi below.
          */
-        add     $sym_offs(.Lbad_efi_msg),%esi   # Error message
+        mov     $sym_offs(.Lbad_efi_msg), %ecx
         xor     %edi,%edi                       # No VGA text buffer
         jmp     .Lprint_err
 .Lget_vtb:
         mov     sym_esi(vga_text_buffer),%edi
 .Lprint_err:
+        add     %ecx, %esi     # Add string offset to relocation base.
+        # NOTE: No further use of sym_esi() till the end of the "function"!
+1:
         lodsb
         test    %al,%al        # Terminate on '\0' sentinel
         je      .Lhalt
@@ -189,11 +192,11 @@ not_multiboot:
         mov     %bl,%al
         out     %al,%dx        # Send a character over the serial line
         test    %edi,%edi      # Is the VGA text buffer available?
-        jz      .Lprint_err
+        jz      1b
         stosb                  # Write a character to the VGA text buffer
         mov     $7,%al
         stosb                  # Write an attribute to the VGA text buffer
-        jmp     .Lprint_err
+        jmp     1b
 .Lhalt: hlt
         jmp     .Lhalt