[xen staging] x86/elf: Remove ASM_CALL_CONSTRAINT from elf_core_save_regs()

[patchbot@xxxxxxx]

commit b9ad1f04bafc4f1b2f355a7ed9853add0a7c7448
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Mar 25 17:55:33 2025 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Mar 26 11:54:59 2025 +0000

    x86/elf: Remove ASM_CALL_CONSTRAINT from elf_core_save_regs()
    
    I was mistaken about when ASM_CALL_CONSTRAINT is applicable.  It is not
    applicable for plain pushes/pops, so remove it from the flags logic.
    
    Clarify the description of ASM_CALL_CONSTRAINT to be explicit about 
unwinding
    using framepointers.
    
    Fixes: 0754534b8a38 ("x86/elf: Improve code generation in 
elf_core_save_regs()")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/include/asm/asm_defns.h  | 5 +++--
 xen/arch/x86/include/asm/x86_64/elf.h | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/asm_defns.h 
b/xen/arch/x86/include/asm/asm_defns.h
index 92b4116a15..72a0082d31 100644
--- a/xen/arch/x86/include/asm/asm_defns.h
+++ b/xen/arch/x86/include/asm/asm_defns.h
@@ -28,8 +28,9 @@ asm ( "\t.equ CONFIG_INDIRECT_THUNK, "
 
 /*
  * This output constraint should be used for any inline asm which has a "call"
- * instruction.  Otherwise the asm may be inserted before the frame pointer
- * gets set up by the containing function.
+ * instruction, which forces the frame pointer to be set up prior to the asm
+ * block.  This matters when unwinding using frame pointers, where the asm's
+ * function can get skipped over.
  */
 #ifdef CONFIG_FRAME_POINTER
 register unsigned long current_stack_pointer asm("rsp");
diff --git a/xen/arch/x86/include/asm/x86_64/elf.h 
b/xen/arch/x86/include/asm/x86_64/elf.h
index f33be46dde..e7bec7327a 100644
--- a/xen/arch/x86/include/asm/x86_64/elf.h
+++ b/xen/arch/x86/include/asm/x86_64/elf.h
@@ -56,7 +56,7 @@ static inline void elf_core_save_regs(ELF_Gregset *core_regs,
     /* orig_rax not filled in for now */
     asm ( "lea (%%rip), %0" : "=r" (core_regs->rip) );
     asm ( "mov %%cs, %0" : "=m" (core_regs->cs) );
-    asm ( "pushfq; popq %0" : "=m" (core_regs->rflags) ASM_CALL_CONSTRAINT );
+    asm ( "pushfq; popq %0" : "=m" (core_regs->rflags) );
     asm ( "movq %%rsp, %0" : "=m" (core_regs->rsp) );
     asm ( "mov %%ss, %0" : "=m" (core_regs->ss) );
     rdmsrl(MSR_FS_BASE, core_regs->thread_fs);
--
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