[PATCH v2] x86/PV: avoid indirect call for I/O emulation quirk hook

[Jan Beulich <jbeulich@xxxxxxxx>]

This way ioemul_handle_proliant_quirk() won't need ENDBR anymore.

While touching this code, also
- arrange for it to not be built at all when !PV,
- add "const" to the last function parameter and bring the definition
  in sync with the declaration (for Misra).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Obviously the file may want moving to pv/ then. I wasn't sure whether
to also fold doing so right into here.

Of course things could also be done the other way around: Have
ioemul_handle_quirk as function name and ioemul_handle_proliant_quirk
be the boolean.
---
v2: Don't use alternative_call(); drop indirect call altogether.

--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -45,7 +45,7 @@ obj-$(CONFIG_LIVEPATCH) += alternative.o
 obj-y += msi.o
 obj-y += msr.o
 obj-$(CONFIG_INDIRECT_THUNK) += indirect-thunk.o
-obj-y += ioport_emulate.o
+obj-$(CONFIG_PV) += ioport_emulate.o
 obj-y += irq.o
 obj-$(CONFIG_KEXEC) += machine_kexec.o
 obj-y += mm.o x86_64/mm.o
--- a/xen/arch/x86/include/asm/io.h
+++ b/xen/arch/x86/include/asm/io.h
@@ -47,10 +47,14 @@ __OUT(b,"b",char)
 __OUT(w,"w",short)
 __OUT(l,,int)
 
-/* Function pointer used to handle platform specific I/O port emulation. */
+/*
+ * Boolean indicator and function used to handle platform specific I/O port
+ * emulation.
+ */
 #define IOEMUL_QUIRK_STUB_BYTES 9
+extern bool ioemul_handle_quirk;
 struct cpu_user_regs;
-extern unsigned int (*ioemul_handle_quirk)(
-    uint8_t opcode, char *io_emul_stub, struct cpu_user_regs *regs);
+unsigned int ioemul_handle_proliant_quirk(
+    uint8_t opcode, char *io_emul_stub, const struct cpu_user_regs *regs);
 
 #endif
--- a/xen/arch/x86/ioport_emulate.c
+++ b/xen/arch/x86/ioport_emulate.c
@@ -8,11 +8,10 @@
 #include <xen/sched.h>
 #include <xen/dmi.h>
 
-unsigned int (*__read_mostly ioemul_handle_quirk)(
-    uint8_t opcode, char *io_emul_stub, struct cpu_user_regs *regs);
+bool __ro_after_init ioemul_handle_quirk;
 
-static unsigned int cf_check ioemul_handle_proliant_quirk(
-    u8 opcode, char *io_emul_stub, struct cpu_user_regs *regs)
+unsigned int ioemul_handle_proliant_quirk(
+    uint8_t opcode, char *io_emul_stub, const struct cpu_user_regs *regs)
 {
     static const char stub[] = {
         0x9c,       /*    pushf           */
@@ -95,7 +94,7 @@ static const struct dmi_system_id __init
 static int __init cf_check ioport_quirks_init(void)
 {
     if ( dmi_check_system(ioport_quirks_tbl) )
-        ioemul_handle_quirk = ioemul_handle_proliant_quirk;
+        ioemul_handle_quirk = true;
 
     return 0;
 }
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -112,7 +112,7 @@ static io_emul_stub_t *io_emul_stub_setu
     /* Some platforms might need to quirk the stub for specific inputs. */
     if ( unlikely(ioemul_handle_quirk) )
     {
-        quirk_bytes = ioemul_handle_quirk(opcode, p, ctxt->ctxt.regs);
+        quirk_bytes = ioemul_handle_proliant_quirk(opcode, p, ctxt->ctxt.regs);
         p += quirk_bytes;
     }