[PATCH v2] x86/entry: shrink insn size for some of our EFLAGS manipulation

[Jan Beulich <jbeulich@suse.com>]

Much like was recently done for setting entry vector, and along the
lines of what we already had in handle_exception_saved, avoid 32-bit
immediates where 8-bit ones do. Reduces .text.entry size by 16 bytes in
my non-CET reference build, while in my CET reference build section size
doesn't change (there and in .text only padding space increases).

Inspired by other long->byte conversion work.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
Numbers above are biased by me also having the straight-line-speculation
change in the tree, thus every JMP is followed by an INT3. Without that,
.text.entry size would also shrink by 16 bytes in the CET build.
---
v2: Drop switch_to_kernel change.

--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -223,7 +223,7 @@ LABEL_LOCAL(.Lrestore_rcx_iret_exit_to_g
 /* No special register assumptions. */
 iret_exit_to_guest:
         andl  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), EFRAME_eflags(%rsp)
-        orl   $X86_EFLAGS_IF, EFRAME_eflags(%rsp)
+        orb   $X86_EFLAGS_IF >> 8, EFRAME_eflags + 1(%rsp)
         addq  $8,%rsp
 .Lft0:  iretq
         _ASM_PRE_EXTABLE(.Lft0, handle_exception)
@@ -346,7 +346,7 @@ LABEL(sysenter_eflags_saved, 0)
 
         GET_STACK_END(bx)
         /* PUSHF above has saved EFLAGS.IF clear (the caller had it set). */
-        orl   $X86_EFLAGS_IF, UREGS_eflags(%rsp)
+        orb   $X86_EFLAGS_IF >> 8, UREGS_eflags + 1(%rsp)
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%rbx), %rcx
         test  %rcx, %rcx
         jz    .Lsyse_cr3_okay
@@ -361,11 +361,11 @@ LABEL(sysenter_eflags_saved, 0)
         cmpb  $0,VCPU_sysenter_disables_events(%rbx)
         movq  VCPU_sysenter_addr(%rbx),%rax
         setne %cl
-        testl $X86_EFLAGS_NT,UREGS_eflags(%rsp)
+        testb $X86_EFLAGS_NT >> 8, UREGS_eflags + 1(%rsp)
         leaq  VCPU_trap_bounce(%rbx),%rdx
 UNLIKELY_START(nz, sysenter_nt_set)
         pushfq
-        andl  $~X86_EFLAGS_NT,(%rsp)
+        andb  $~(X86_EFLAGS_NT >> 8), 1(%rsp)
         popfq
 UNLIKELY_END(sysenter_nt_set)
         testq %rax,%rax