[Xen-devel] [PATCH v4 09/19] xen/arm64: Use alternative to skip the check of pending serrors

[Wei Chen <Wei.Chen@xxxxxxx>]

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 ARM64 supports the alternative patching feature. We can use an
ALTERNATIVE to avoid checking option at every trap. We added a new
cpufeature named "SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT". This feature
will be enabled when the option is not diverse.

Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/arch/arm/arm64/entry.S | 41 +++++++++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index 02802c0..3d2fdfb 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -1,5 +1,6 @@
 #include <asm/asm_defns.h>
 #include <asm/regs.h>
+#include <asm/alternative.h>
 #include <public/xen.h>
 
 /*
@@ -229,12 +230,14 @@ hyp_irq:
 
 guest_sync:
         entry   hyp=0, compat=0
-        bl      check_pending_vserror
         /*
-         * If x0 is Non-zero, a vSError took place, the initial exception
-         * doesn't have any significance to be handled. Exit ASAP
+         * The vSError will be checked while SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
+         * is not set. If a vSError took place, the initial exception will be
+         * skipped. Exit ASAP
          */
-        cbnz    x0, 1f
+        ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
+                    "nop; nop",
+                    SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
         msr     daifclr, #2
         mov     x0, sp
         bl      do_trap_hypervisor
@@ -243,12 +246,14 @@ guest_sync:
 
 guest_irq:
         entry   hyp=0, compat=0
-        bl      check_pending_vserror
         /*
-         * If x0 is Non-zero, a vSError took place, the initial exception
-         * doesn't have any significance to be handled. Exit ASAP
+         * The vSError will be checked while SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
+         * is not set. If a vSError took place, the initial exception will be
+         * skipped. Exit ASAP
          */
-        cbnz    x0, 1f
+        ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
+                    "nop; nop",
+                    SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
         mov     x0, sp
         bl      do_trap_irq
 1:
@@ -267,12 +272,14 @@ guest_error:
 
 guest_sync_compat:
         entry   hyp=0, compat=1
-        bl      check_pending_vserror
         /*
-         * If x0 is Non-zero, a vSError took place, the initial exception
-         * doesn't have any significance to be handled. Exit ASAP
+         * The vSError will be checked while SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
+         * is not set. If a vSError took place, the initial exception will be
+         * skipped. Exit ASAP
          */
-        cbnz    x0, 1f
+        ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
+                    "nop; nop",
+                    SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
         msr     daifclr, #2
         mov     x0, sp
         bl      do_trap_hypervisor
@@ -281,12 +288,14 @@ guest_sync_compat:
 
 guest_irq_compat:
         entry   hyp=0, compat=1
-        bl      check_pending_vserror
         /*
-         * If x0 is Non-zero, a vSError took place, the initial exception
-         * doesn't have any significance to be handled. Exit ASAP
+         * The vSError will be checked while SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
+         * is not set. If a vSError took place, the initial exception will be
+         * skipped. Exit ASAP
          */
-        cbnz    x0, 1f
+        ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
+                    "nop; nop",
+                    SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
         mov     x0, sp
         bl      do_trap_irq
 1:
-- 
2.7.4


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