[Xen-devel] [PATCH v2 07/15] xen/arm: p2m: Switch the p2m lock from spinlock to rwlock

[Julien Grall <julien.grall@xxxxxxx>]

P2M reads do not require to be serialized. This will add contention
when PV drivers are using multi-queue because parallel grant
map/unmaps/copies will happen on DomU's p2m.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

---
    I have not done benchark to verify the performance, however a rwlock
    is always an improvement compare to a spinlock when most of the
    access only read data.

    It might be possible to convert the rwlock to a per-cpu rwlock which
    show some improvement on x86.

    Changes in v2:
        - Add Stefano's reviewed-by
---
 xen/arch/arm/p2m.c        | 12 ++++++------
 xen/include/asm-arm/p2m.h |  3 ++-
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 5c67090..9ba8904 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -49,27 +49,27 @@ static bool_t p2m_mapping(lpae_t pte)
 
 static inline void p2m_write_lock(struct p2m_domain *p2m)
 {
-    spin_lock(&p2m->lock);
+    write_lock(&p2m->lock);
 }
 
 static inline void p2m_write_unlock(struct p2m_domain *p2m)
 {
-    spin_unlock(&p2m->lock);
+    write_unlock(&p2m->lock);
 }
 
 static inline void p2m_read_lock(struct p2m_domain *p2m)
 {
-    spin_lock(&p2m->lock);
+    read_lock(&p2m->lock);
 }
 
 static inline void p2m_read_unlock(struct p2m_domain *p2m)
 {
-    spin_unlock(&p2m->lock);
+    read_unlock(&p2m->lock);
 }
 
 static inline int p2m_is_locked(struct p2m_domain *p2m)
 {
-    return spin_is_locked(&p2m->lock);
+    return rw_is_locked(&p2m->lock);
 }
 
 void p2m_dump_info(struct domain *d)
@@ -1388,7 +1388,7 @@ int p2m_init(struct domain *d)
     struct p2m_domain *p2m = &d->arch.p2m;
     int rc = 0;
 
-    spin_lock_init(&p2m->lock);
+    rwlock_init(&p2m->lock);
     INIT_PAGE_LIST_HEAD(&p2m->pages);
 
     p2m->vmid = INVALID_VMID;
diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
index 20a220ea..abda70c 100644
--- a/xen/include/asm-arm/p2m.h
+++ b/xen/include/asm-arm/p2m.h
@@ -3,6 +3,7 @@
 
 #include <xen/mm.h>
 #include <xen/radix-tree.h>
+#include <xen/rwlock.h>
 #include <public/vm_event.h> /* for vm_event_response_t */
 #include <public/memory.h>
 #include <xen/p2m-common.h>
@@ -20,7 +21,7 @@ extern void memory_type_changed(struct domain *);
 /* Per-p2m-table state */
 struct p2m_domain {
     /* Lock that protects updates to the p2m */
-    spinlock_t lock;
+    rwlock_t lock;
 
     /* Pages used to construct the p2m */
     struct page_list_head pages;
-- 
1.9.1


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