[PATCH 2/4] x86: reduce CET-SS related #ifdef-ary

[Jan Beulich <jbeulich@xxxxxxxx>]

Commit b586a81b7a90 ("x86/CET: Fix build following c/s 43b98e7190") had
to introduce a number of #ifdef-s to make the build work with older tool
chains. Introduce an assembler macro covering for tool chains not
knowing of CET-SS, allowing some conditionals where just SETSSBSY is the
problem to be dropped again.

No change to generated code.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Now that I've done this I'm not longer sure which direction is better to
follow: On one hand this introduces dead code (even if just NOPs) into
CET-SS-disabled builds. Otoh this is a step towards breaking the tool
chain version dependency of the feature.

I've also dropped conditionals around bigger chunks of code; while I
think that's preferable, I'm open to undo those parts.

--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -31,7 +31,6 @@ ENTRY(__high_start)
         jz      .L_bsp
 
         /* APs.  Set up shadow stacks before entering C. */
-#ifdef CONFIG_XEN_SHSTK
         testl   $cpufeat_mask(X86_FEATURE_XEN_SHSTK), \
                 CPUINFO_FEATURE_OFFSET(X86_FEATURE_XEN_SHSTK) + 
boot_cpu_data(%rip)
         je      .L_ap_shstk_done
@@ -55,7 +54,6 @@ ENTRY(__high_start)
         mov     $XEN_MINIMAL_CR4 | X86_CR4_CET, %ecx
         mov     %rcx, %cr4
         setssbsy
-#endif
 
 .L_ap_shstk_done:
         call    start_secondary
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -668,7 +668,7 @@ static void __init noreturn reinit_bsp_s
     stack_base[0] = stack;
     memguard_guard_stack(stack);
 
-    if ( IS_ENABLED(CONFIG_XEN_SHSTK) && cpu_has_xen_shstk )
+    if ( cpu_has_xen_shstk )
     {
         wrmsrl(MSR_PL0_SSP,
                (unsigned long)stack + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 
8);
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -198,9 +198,7 @@ ENTRY(cr4_pv32_restore)
 
 /* See lstar_enter for entry register state. */
 ENTRY(cstar_enter)
-#ifdef CONFIG_XEN_SHSTK
         ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
-#endif
         /* sti could live here when we don't switch page tables below. */
         CR4_PV32_RESTORE
         movq  8(%rsp),%rax /* Restore %rax. */
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -237,9 +237,7 @@ iret_exit_to_guest:
  * %ss must be saved into the space left by the trampoline.
  */
 ENTRY(lstar_enter)
-#ifdef CONFIG_XEN_SHSTK
         ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
-#endif
         /* sti could live here when we don't switch page tables below. */
         movq  8(%rsp),%rax /* Restore %rax. */
         movq  $FLAT_KERNEL_SS,8(%rsp)
@@ -273,9 +271,7 @@ ENTRY(lstar_enter)
         jmp   test_all_events
 
 ENTRY(sysenter_entry)
-#ifdef CONFIG_XEN_SHSTK
         ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
-#endif
         /* sti could live here when we don't switch page tables below. */
         pushq $FLAT_USER_SS
         pushq $0
--- a/xen/include/asm-x86/asm-defns.h
+++ b/xen/include/asm-x86/asm-defns.h
@@ -7,3 +7,9 @@
     .byte 0x0f, 0x01, 0xcb
 .endm
 #endif
+
+#ifndef CONFIG_HAS_AS_CET_SS
+.macro setssbsy
+    .byte 0xf3, 0x0f, 0x01, 0xe8
+.endm
+#endif