[xen master] x86/kexec: Fix and expand comments for kexec_reloc()

[patchbot@xxxxxxx]

commit 5cdae3c505f3c0e9c261e19a5b3733161e696603
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Mar 17 19:44:56 2026 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jun 23 19:15:51 2026 +0100

    x86/kexec: Fix and expand comments for kexec_reloc()
    
    The order of shutdown is delicate.  Explain things a little better.
    
    Fix two comments about leaving Long Mode.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/x86/x86_64/kexec_reloc.S | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/x86_64/kexec_reloc.S 
b/xen/arch/x86/x86_64/kexec_reloc.S
index b52d31a654..d0951ea1e1 100644
--- a/xen/arch/x86/x86_64/kexec_reloc.S
+++ b/xen/arch/x86/x86_64/kexec_reloc.S
@@ -27,6 +27,14 @@
         .section .text.kexec, "ax", @progbits
         .code64
 
+        /*
+         * kexec_reloc() is entered in it's natural position within Xen.
+         *
+         * A copy of .text.kexec is identity mapped at %rdi within the
+         * pagetables in %rsi.  Xen's .text is mapped into %rsi at it's high
+         * alias allowing to pivot, but most other things including the stacks
+         * are not mapped.
+         */
 FUNC(kexec_reloc, PAGE_SIZE)
         /* %rdi - code page maddr */
         /* %rsi - page table maddr */
@@ -36,10 +44,17 @@ FUNC(kexec_reloc, PAGE_SIZE)
 
         movq    %rcx, %rbp
 
-        /* Setup stack. */
+        /*
+         * Move to the identity mapped stack.
+         *
+         * Note this mapping doesn't exist until the pagetable switch.
+         */
         leaq    (.Lreloc_stack_base - kexec_reloc)(%rdi), %rsp
 
-        /* Load reloc page table. */
+        /*
+         * Move to the relocation pagetables.  Xen's .text is mapped allowing
+         * the pivot, but very little else is.
+         */
         movq    %rsi, %cr3
 
         /* Jump to identity mapped code. */
@@ -153,12 +168,12 @@ FUNC_LOCAL(compatibility_mode)
         movl    %eax, %gs
         movl    %eax, %ss
 
-        /* Disable paging and therefore leave 64 bit mode. */
+        /* Disable paging and therefore leave long mode. */
         movl    %cr0, %eax
         andl    $~X86_CR0_PG, %eax
         movl    %eax, %cr0
 
-        /* Disable long mode */
+        /* Clear EFER.LME */
         movl    $MSR_EFER, %ecx
         rdmsr
         andl    $~EFER_LME, %eax
--
generated by git-patchbot for /home/xen/git/xen.git#master