[PATCH] x86/acrn: Improve ACRN hypercalls

[Uros Bizjak]

As explained in section 6.47.5.2, "Specifying Registers for Local Variables"
of the GCC info documentation, the correct way to specify register for
input operands when calling Extended 'asm' is to define a local register
variable and associate it with a specified register:

	register unsigned long r8 asm ("r8") = hcall_id;

Use the above approach instead of explicit MOV to R8 at the beginning
of the asm. The relaxed assignment allows compiler to optimize and
shrink drivers/virt/acrn.o for 181 bytes:

   text    data     bss     dec     hex filename
   4284     208       0    4492    118c hsm-new.o
   4465     208       0    4673    1241 hsm-old.o

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
 arch/x86/include/asm/acrn.h | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/acrn.h b/arch/x86/include/asm/acrn.h
index e003a01b7c67..601867085b95 100644
--- a/arch/x86/include/asm/acrn.h
+++ b/arch/x86/include/asm/acrn.h
@@ -29,19 +29,16 @@ static inline u32 acrn_cpuid_base(void)
  *   - Hypercall number is passed in R8 register.
  *   - Up to 2 arguments are passed in RDI, RSI.
  *   - Return value will be placed in RAX.
- *
- * Because GCC doesn't support R8 register as direct register constraints, use
- * supported constraint as input with a explicit MOV to R8 in beginning of asm.
  */
 static inline long acrn_hypercall0(unsigned long hcall_id)
 {
 	long result;
 
-	asm volatile("movl %1, %%r8d\n\t"
-		     "vmcall\n\t"
+	register unsigned long r8 asm ("r8") = hcall_id;
+	asm volatile("vmcall"
 		     : "=a" (result)
-		     : "g" (hcall_id)
-		     : "r8", "memory");
+		     : "r" (r8)
+		     : "memory");
 
 	return result;
 }
@@ -51,11 +48,11 @@ static inline long acrn_hypercall1(unsigned long hcall_id,
 {
 	long result;
 
-	asm volatile("movl %1, %%r8d\n\t"
-		     "vmcall\n\t"
+	register unsigned long r8 asm ("r8") = hcall_id;
+	asm volatile("vmcall"
 		     : "=a" (result)
-		     : "g" (hcall_id), "D" (param1)
-		     : "r8", "memory");
+		     : "r" (r8), "D" (param1)
+		     : "memory");
 
 	return result;
 }
@@ -66,11 +63,11 @@ static inline long acrn_hypercall2(unsigned long hcall_id,
 {
 	long result;
 
-	asm volatile("movl %1, %%r8d\n\t"
-		     "vmcall\n\t"
+	register unsigned long r8 asm ("r8") = hcall_id;
+	asm volatile("vmcall"
 		     : "=a" (result)
-		     : "g" (hcall_id), "D" (param1), "S" (param2)
-		     : "r8", "memory");
+		     : "r" (r8), "D" (param1), "S" (param2)
+		     : "memory");
 
 	return result;
 }
-- 
2.37.1