[PATCH for-4.21 2/3] xen/console: Optimise parameter order of vprintk_common()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

For ABIs which pass parameters by register (all cases that we compile Xen
for), inserting new arguments on the left hand side involves shuffling all
other parameters along by one register whereas appending a new argument
doesn't involve shuffling of existing registers.

Reorder vprintk_common()'s prefix parameter to being last.  This is a marginal
improvement on all architectures:

  Function                              old     new   delta
  vprintk                                18      12      -6  x86
  vprintk                                32      24      -8  arm32
  vprintk                                52      48      -4  arm64
  vprintk                                52      48      -4  riscv64
  vprintk                                80      72      -8  ppc64

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

This taken from a security series, hence partially reviewed already.
---
 xen/drivers/char/console.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 2926c97df9a4..f98142d9b9b9 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -910,7 +910,7 @@ static void printk_start_of_line(const char *prefix)
     __putstr(tstr);
 }
 
-static void vprintk_common(const char *prefix, const char *fmt, va_list args)
+static void vprintk_common(const char *fmt, va_list args, const char *prefix)
 {
     struct vps {
         bool continued, do_print;
@@ -964,7 +964,7 @@ static void vprintk_common(const char *prefix, const char 
*fmt, va_list args)
 
 void vprintk(const char *fmt, va_list args)
 {
-    vprintk_common("(XEN) ", fmt, args);
+    vprintk_common(fmt, args, "(XEN) ");
 }
 
 void printk(const char *fmt, ...)
@@ -984,7 +984,7 @@ void guest_printk(const struct domain *d, const char *fmt, 
...)
     snprintf(prefix, sizeof(prefix), "(d%d) ", d->domain_id);
 
     va_start(args, fmt);
-    vprintk_common(prefix, fmt, args);
+    vprintk_common(fmt, args, prefix);
     va_end(args);
 }
 
-- 
2.39.5