[xen stable-4.16] x86: fix early boot output

[patchbot@xxxxxxx]

commit 6ede73cab9ec9cf1ad0be0224279b645b84d1a9a
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Aug 3 16:32:28 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 3 16:32:28 2023 +0200

    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@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: b1c16800e52743d9afd9af62c810f03af16dd942
    master date: 2023-07-19 10:22:56 +0200
---
 xen/arch/x86/boot/head.S | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index dd1bea0d10..e1b05a2557 100644
--- 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
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.16