All other SPEC_CTRL_{ENTRY,EXIT}_* helpers hold stack_end in %r14.  Adjust it
for consistency, freeing up %rbx to be used differently.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
---
 xen/arch/x86/include/asm/spec_ctrl_asm.h | 8 ++++----
 xen/arch/x86/x86_64/entry.S              | 8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/include/asm/spec_ctrl_asm.h b/xen/arch/x86/include/asm/spec_ctrl_asm.h
index 77f6c35bb9c5..acdb526d292d 100644
--- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
+++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
@@ -344,10 +344,10 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
  */
 .macro SPEC_CTRL_EXIT_TO_XEN
 /*
- * Requires %rbx=stack_end
+ * Requires %r14=stack_end
  * Clobbers %rax, %rcx, %rdx
  */
-    testb $SCF_ist_sc_msr, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%rbx)
+    testb $SCF_ist_sc_msr, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
     jz .L\@_skip_sc_msr
 
     /*
@@ -358,10 +358,10 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
      */
     xor %edx, %edx
 
-    testb $SCF_use_shadow, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%rbx)
+    testb $SCF_use_shadow, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
     jz .L\@_skip_sc_msr
 
-    mov STACK_CPUINFO_FIELD(shadow_spec_ctrl)(%rbx), %eax
+    mov STACK_CPUINFO_FIELD(shadow_spec_ctrl)(%r14), %eax
     mov $MSR_SPEC_CTRL, %ecx
     wrmsr
 
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index a1c860f56949..525877e97330 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -665,15 +665,15 @@ restore_all_xen:
          * Check whether we need to switch to the per-CPU page tables, in
          * case we return to late PV exit code (from an NMI or #MC).
          */
-        GET_STACK_END(bx)
-        cmpb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%rbx)
+        GET_STACK_END(14)
+        cmpb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
 UNLIKELY_START(ne, exit_cr3)
-        mov   STACK_CPUINFO_FIELD(pv_cr3)(%rbx), %rax
+        mov   STACK_CPUINFO_FIELD(pv_cr3)(%r14), %rax
         mov   %rax, %cr3
 UNLIKELY_END(exit_cr3)
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        SPEC_CTRL_EXIT_TO_XEN     /* Req: %rbx=end, Clob: acd */
+        SPEC_CTRL_EXIT_TO_XEN     /* Req: %r14=end, Clob: acd */
 
         RESTORE_ALL adj=8
         iretq
-- 
2.30.2

On 13.09.2023 22:27, Andrew Cooper wrote:
> All other SPEC_CTRL_{ENTRY,EXIT}_* helpers hold stack_end in %r14.  Adjust it
> for consistency, freeing up %rbx to be used differently.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>

The choice of %rbx was, iirc, for avoiding unnecessary code size increase
(due to extra REX prefixes needed in some of the affected insns). I
understand you view this as not a real concern, but I think saying so
explicitly in the description would be at least desirable. After all there
would also be the alternative of further parametrizing the involved macros.

Jan

On 14/09/2023 9:51 am, Jan Beulich wrote:
> On 13.09.2023 22:27, Andrew Cooper wrote:
>> All other SPEC_CTRL_{ENTRY,EXIT}_* helpers hold stack_end in %r14.  Adjust it
>> for consistency, freeing up %rbx to be used differently.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>
> The choice of %rbx was, iirc, for avoiding unnecessary code size increase
> (due to extra REX prefixes needed in some of the affected insns). I
> understand you view this as not a real concern, but I think saying so
> explicitly in the description would be at least desirable. After all there
> would also be the alternative of further parametrizing the involved macros.

Yeah I'm seriously not concerned about a REX prefix or two.

This is about making all the helpers more consistent, to cut down on the
number of errors involved in modifying this mess.

~Andrew