[Xen-devel] [PATCH V4 10/11] xen/arm: io: Use binary search for mmio handler lookup

[Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>]

As the number of I/O handlers increase, the overhead associated with
linear lookup also increases. The system might have maximum of 144
(assuming CONFIG_NR_CPUS=128) mmio handlers. In worst case scenario,
it would require 144 iterations for finding a matching handler. Now
it is time for us to change from linear (complexity O(n)) to a binary
search (complexity O(log n) for reducing mmio handler lookup overhead.

Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
---
Changes since v3:
  Moved the function bsearch() to common file xen/common/bsearch.c.

Changes since v2:
  Converted mmio lookup code to a critical section.
  Copied the function bsreach() from Linux kernel.

 xen/arch/arm/io.c | 42 +++++++++++++++++++++++++-----------------
 1 file changed, 25 insertions(+), 17 deletions(-)

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 40330f0..0471ba8 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -20,6 +20,8 @@
 #include <xen/lib.h>
 #include <xen/spinlock.h>
 #include <xen/sched.h>
+#include <xen/sort.h>
+#include <xen/bsearch.h>
 #include <asm/current.h>
 #include <asm/mmio.h>
 
@@ -70,27 +72,29 @@ static int handle_write(const struct mmio_handler *handler, 
struct vcpu *v,
                                handler->priv);
 }
 
-static const struct mmio_handler *find_mmio_handler(struct domain *d,
-                                                    paddr_t gpa)
+static int cmp_mmio_handler(const void *key, const void *elem)
 {
-    const struct mmio_handler *handler;
-    unsigned int i;
-    struct vmmio *vmmio = &d->arch.vmmio;
+    const struct mmio_handler *handler0 = key;
+    const struct mmio_handler *handler1 = elem;
 
-    read_lock(&vmmio->lock);
+    if ( handler0->addr < handler1->addr )
+        return -1;
 
-    for ( i = 0; i < vmmio->num_entries; i++ )
-    {
-        handler = &vmmio->handlers[i];
+    if ( handler0->addr > (handler1->addr + handler1->size) )
+        return 1;
 
-        if ( (gpa >= handler->addr) &&
-             (gpa < (handler->addr + handler->size)) )
-            break;
-    }
+    return 0;
+}
 
-    if ( i == vmmio->num_entries )
-        handler = NULL;
+static const struct mmio_handler *find_mmio_handler(struct vcpu *v, paddr_t 
gpa)
+{
+    struct vmmio *vmmio = &v->domain->arch.vmmio;
+    struct mmio_handler key = {.addr = gpa};
+    const struct mmio_handler *handler;
 
+    read_lock(&vmmio->lock);
+    handler = bsearch(&key, vmmio->handlers, vmmio->num_entries,
+                      sizeof(*handler), cmp_mmio_handler);
     read_unlock(&vmmio->lock);
 
     return handler;
@@ -99,9 +103,9 @@ static const struct mmio_handler *find_mmio_handler(struct 
domain *d,
 int handle_mmio(mmio_info_t *info)
 {
     struct vcpu *v = current;
-    const struct mmio_handler *handler = NULL;
+    const struct mmio_handler *handler;
 
-    handler = find_mmio_handler(v->domain, info->gpa);
+    handler = find_mmio_handler(v, info->gpa);
     if ( !handler )
         return 0;
 
@@ -131,6 +135,10 @@ void register_mmio_handler(struct domain *d,
 
     vmmio->num_entries++;
 
+    /* Sort mmio handlers in ascending order based on base address */
+    sort(vmmio->handlers, vmmio->num_entries, sizeof(struct mmio_handler),
+        cmp_mmio_handler, NULL);
+
     write_unlock(&vmmio->lock);
 }
 
-- 
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc. 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
a Linux Foundation Collaborative Project


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