[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>

4 files changed, 13 insertions, 0 deletions

diff --git a/include/asm-ia64/asmmacro.h b/include/asm-ia64/asmmacro.h
index c22b4658fc61..c1642fd64029 100644
--- a/include/asm-ia64/asmmacro.h
+++ b/include/asm-ia64/asmmacro.h
@@ -104,6 +104,16 @@ name:
 #endif
 
 /*
+ * If physical stack register size is different from DEF_NUM_STACK_REG,
+ * dynamically patch the kernel for correct size.
+ */
+	.section ".data.patch.phys_stack_reg", "a"
+	.previous
+#define LOAD_PHYS_STACK_REG_SIZE(reg)			\
+[1:]	adds reg=IA64_NUM_PHYS_STACK_REG*8+8,r0;	\
+	.xdata4 ".data.patch.phys_stack_reg", 1b-.
+
+/*
  * Up until early 2004, use of .align within a function caused bad unwind info.
  * TEXT_ALIGN(n) expands into ".align n" if a fixed GAS is available or into nothing
  * otherwise.
diff --git a/include/asm-ia64/patch.h b/include/asm-ia64/patch.h
index 4797f3535e6d..a71543084fb4 100644
--- a/include/asm-ia64/patch.h
+++ b/include/asm-ia64/patch.h
@@ -20,6 +20,7 @@ extern void ia64_patch_imm60 (u64 insn_addr, u64 val);		/* patch "brl" w/ip-rel
 
 extern void ia64_patch_mckinley_e9 (unsigned long start, unsigned long end);
 extern void ia64_patch_vtop (unsigned long start, unsigned long end);
+extern void ia64_patch_phys_stack_reg(unsigned long val);
 extern void ia64_patch_gate (void);
 
 #endif /* _ASM_IA64_PATCH_H */
diff --git a/include/asm-ia64/processor.h b/include/asm-ia64/processor.h
index 5830d36fd8e6..88c728b9ff45 100644
--- a/include/asm-ia64/processor.h
+++ b/include/asm-ia64/processor.h
@@ -19,6 +19,7 @@
 #include <asm/ptrace.h>
 #include <asm/ustack.h>
 
+#define IA64_NUM_PHYS_STACK_REG	96
 #define IA64_NUM_DBG_REGS	8
 
 #define DEFAULT_MAP_BASE	__IA64_UL_CONST(0x2000000000000000)
diff --git a/include/asm-ia64/sections.h b/include/asm-ia64/sections.h
index e9eb7f62d32b..dc42a359894f 100644
--- a/include/asm-ia64/sections.h
+++ b/include/asm-ia64/sections.h
@@ -11,6 +11,7 @@
 extern char __per_cpu_start[], __per_cpu_end[], __phys_per_cpu_start[];
 extern char __start___vtop_patchlist[], __end___vtop_patchlist[];
 extern char __start___mckinley_e9_bundles[], __end___mckinley_e9_bundles[];
+extern char __start___phys_stack_reg_patchlist[], __end___phys_stack_reg_patchlist[];
 extern char __start_gate_section[];
 extern char __start_gate_mckinley_e9_patchlist[], __end_gate_mckinley_e9_patchlist[];
 extern char __start_gate_vtop_patchlist[], __end_gate_vtop_patchlist[];