[xen master] x86/idt: Minor improvements to _update_gate_addr_lower()

[patchbot@xxxxxxx]

commit cf8fdef99f88cf36e19b065761f8bb8aedd28b92
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Mar 14 13:32:12 2025 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Aug 13 13:13:08 2025 +0100

    x86/idt: Minor improvements to _update_gate_addr_lower()
    
    After some experimentation, using .a/.b makes far better logic than using 
the
    named fields, as both Clang and GCC spill idte to the stack when named 
fields
    are used.
    
    GCC seems to do something very daft for the addr1 field.  It takes addr,
    shifts it by 32, then ANDs with 0xffff0000000000000UL, which requires
    manifesting a MOVABS.
    
    Clang follows the C, whereby it ANDs with $imm32, then shifts, avoiding the
    MOVABS entirely.
    
    No functional change.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/include/asm/idt.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/include/asm/idt.h b/xen/arch/x86/include/asm/idt.h
index f613d5693e..acb3613474 100644
--- a/xen/arch/x86/include/asm/idt.h
+++ b/xen/arch/x86/include/asm/idt.h
@@ -92,15 +92,16 @@ static inline void _set_gate_lower(idt_entry_t *gate, 
unsigned long type,
  * Update the lower half handler of an IDT entry, without changing any other
  * configuration.
  */
-static inline void _update_gate_addr_lower(idt_entry_t *gate, void *addr)
+static inline void _update_gate_addr_lower(idt_entry_t *gate, void *_addr)
 {
+    unsigned long addr = (unsigned long)_addr;
+    unsigned int addr1 = addr & 0xffff0000U; /* GCC: force better codegen. */
     idt_entry_t idte;
-    idte.a = gate->a;
 
-    idte.b = ((unsigned long)(addr) >> 32);
-    idte.a &= 0x0000FFFFFFFF0000ULL;
-    idte.a |= (((unsigned long)(addr) & 0xFFFF0000UL) << 32) |
-        ((unsigned long)(addr) & 0xFFFFUL);
+    idte.b = addr >> 32;
+    idte.a = gate->a & 0x0000ffffffff0000UL;
+    idte.a |= (unsigned long)addr1 << 32;
+    idte.a |= addr & 0xffff;
 
     _write_gate_lower(gate, &idte);
 }
--
generated by git-patchbot for /home/xen/git/xen.git#master