[PATCH v3 10/19] xen/arm32: Use alternative to skip the check of pending serrors

[Wei Chen <Wei.Chen@arm.com>]

We have provided an option to administrator to determine how to
handle the SErrors. In order to skip the check of pending SError,
in conventional way, we have to read the option every time before
we try to check the pending SError. This will add overhead to check
the option at every trap.

The ARM32 supports the alternative patching feature. We can use an
ALTERNATIVE to avoid checking option at every trap. We added a new
cpufeature named "SKIP_CHECK_PENDING_VSERROR". This feature will be
enabled when the option is not diverse.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>

---
Note:
This patch depends on the "xen/arm32: Introduce alternative runtime
patching" [1] which is still in review.

[1]https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg04333.html
---
 xen/arch/arm/arm32/entry.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S
index 2187226..105cae8 100644
--- a/xen/arch/arm/arm32/entry.S
+++ b/xen/arch/arm/arm32/entry.S
@@ -1,5 +1,6 @@
 #include <asm/asm_defns.h>
 #include <asm/regs.h>
+#include <asm/alternative.h>
 #include <public/xen.h>
 
 #define SAVE_ONE_BANKED(reg)    mrs r11, reg; str r11, [sp, #UREGS_##reg]
@@ -44,6 +45,14 @@ save_guest_regs:
         SAVE_BANKED(fiq)
         SAVE_ONE_BANKED(R8_fiq); SAVE_ONE_BANKED(R9_fiq); SAVE_ONE_BANKED(R10_fiq)
         SAVE_ONE_BANKED(R11_fiq); SAVE_ONE_BANKED(R12_fiq);
+
+        /*
+         * If the SKIP_CHECK_PENDING_VSERROR has been set in the cpu feature,
+         * the checking of pending SErrors will be skipped.
+         */
+        ALTERNATIVE("nop",
+                    "b skip_check",
+                    SKIP_CHECK_PENDING_VSERROR)
         /*
          * Start to check pending virtual abort in the gap of Guest -> HYP
          * world switch.
@@ -99,6 +108,7 @@ abort_guest_exit_end:
          */
         bne return_from_trap
 
+skip_check:
         mov pc, lr
 
 #define DEFINE_TRAP_ENTRY(trap)                                         \
-- 
2.7.4


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