[PATCH v2 1/6] memory: reintroduce BQL-free fine-grained PIO/MMIO

[Igor Mammedov]

This patch brings back Jan's idea [1] of BQL-free IO access

This will let us make access to ACPI PM/HPET timers cheaper,
and prevent BQL contention in case of workload that heavily
uses the timers with a lot of vCPUs.

1) 196ea13104f (memory: Add global-locking property to memory regions)
   ... de7ea885c539 (kvm: Switch to unlocked MMIO)

Signed-off-by: Igor Mammedov <imamm...@redhat.com>
---
 include/system/memory.h | 10 ++++++++++
 system/memory.c         |  6 ++++++
 system/physmem.c        |  2 +-
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/system/memory.h b/include/system/memory.h
index e2cd6ed126..d04366c994 100644
--- a/include/system/memory.h
+++ b/include/system/memory.h
@@ -833,6 +833,7 @@ struct MemoryRegion {
     bool nonvolatile;
     bool rom_device;
     bool flush_coalesced_mmio;
+    bool lockless_io;
     bool unmergeable;
     uint8_t dirty_log_mask;
     bool is_iommu;
@@ -2341,6 +2342,15 @@ void memory_region_set_flush_coalesced(MemoryRegion *mr);
  */
 void memory_region_clear_flush_coalesced(MemoryRegion *mr);
 
+/**
+ * memory_region_enable_lockless_io: Enable lockless (BQL free) acceess.
+ *
+ * Enable BQL-free access for devices with fine-grained locking.
+ *
+ * @mr: the memory region to be updated.
+ */
+void memory_region_enable_lockless_io(MemoryRegion *mr);
+
 /**
  * memory_region_add_eventfd: Request an eventfd to be triggered when a word
  *                            is written to a location.
diff --git a/system/memory.c b/system/memory.c
index 5646547940..9a5a262112 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -2546,6 +2546,12 @@ void memory_region_clear_flush_coalesced(MemoryRegion 
*mr)
     }
 }
 
+void memory_region_enable_lockless_io(MemoryRegion *mr)
+{
+    mr->lockless_io = true;
+    mr->disable_reentrancy_guard = true;
+}
+
 void memory_region_add_eventfd(MemoryRegion *mr,
                                hwaddr addr,
                                unsigned size,
diff --git a/system/physmem.c b/system/physmem.c
index 130c148ffb..107871e2b3 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2909,7 +2909,7 @@ bool prepare_mmio_access(MemoryRegion *mr)
 {
     bool release_lock = false;
 
-    if (!bql_locked()) {
+    if (!bql_locked() && !mr->lockless_io) {
         bql_lock();
         release_lock = true;
     }
-- 
2.47.1