[IA64] remove per-cpu ia64_phys_stacked_size_p8

It's not efficient to use a per-cpu variable just to store
how many physical stack register a cpu has.  Ever since the
incarnation of ia64 up till upcoming Montecito processor, that
variable has "glued" to 96. Having a variable in memory means
that the kernel is burning an extra cacheline access on every
syscall and kernel exit path.  Such "static" value is better
served with the instruction patching utility exists today.
Convert ia64_phys_stacked_size_p8 into dynamic insn patching.

This also has a pleasant side effect of eliminating access to
per-cpu area while psr.ic=0 in the kernel exit path. (fixable
for per-cpu DTC work, but why bother?)

There are some concerns with the default value that the instruc-
tion encoded in the kernel image.  It shouldn't be concerned.
The reasons are:

(1) cpu_init() is called at CPU initialization.  In there, we
    find out physical stack register size from PAL and patch
    two instructions in kernel exit code.  The code in question
    can not be executed before the patching is done.

(2) current implementation stores zero in ia64_phys_stacked_size_p8,
    and that's what the current kernel exit path loads the value with.
    With the new code, it is equivalent that we store reg size 96
    in ia64_phys_stacked_size_p8, thus creating a better safety net.
    Given (1) above can never fail, having (2) is just a bonus.

All in all, this patch allow one less memory reference in the kernel
exit path, thus reducing syscall and interrupt return latency; and
avoid polluting potential useful data in the CPU cache.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/ia64/kernel/setup.c b/arch/ia64/kernel/setup.c
index ad567b8d432e..f167b89f24eb 100644
--- a/arch/ia64/kernel/setup.c
+++ b/arch/ia64/kernel/setup.c
@@ -75,7 +75,6 @@ extern void ia64_setup_printk_clock(void);
 
 DEFINE_PER_CPU(struct cpuinfo_ia64, cpu_info);
 DEFINE_PER_CPU(unsigned long, local_per_cpu_offset);
-DEFINE_PER_CPU(unsigned long, ia64_phys_stacked_size_p8);
 unsigned long ia64_cycles_per_usec;
 struct ia64_boot_param *ia64_boot_param;
 struct screen_info screen_info;
@@ -836,6 +835,7 @@ void __cpuinit
 cpu_init (void)
 {
         extern void __cpuinit ia64_mmu_init (void *);
+        static unsigned long max_num_phys_stacked = IA64_NUM_PHYS_STACK_REG;
         unsigned long num_phys_stacked;
         pal_vm_info_2_u_t vmi;
         unsigned int max_ctx;
@@ -949,7 +949,10 @@ cpu_init (void)
                 num_phys_stacked = 96;
         }
         /* size of physical stacked register partition plus 8 bytes: */
-        __get_cpu_var(ia64_phys_stacked_size_p8) = num_phys_stacked*8 + 8;
+        if (num_phys_stacked > max_num_phys_stacked) {
+                ia64_patch_phys_stack_reg(num_phys_stacked*8 + 8);
+                max_num_phys_stacked = num_phys_stacked;
+        }
         platform_cpu_init();
         pm_idle = default_idle;
 }