memory: reintroduce BQL-free fine-grained PIO/MMIO

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 <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20250814160600.2327672-2-imammedo@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/system/memory.h b/include/system/memory.h
index e2cd6ed126..aa85fc27a1 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;
@@ -2342,6 +2343,17 @@ 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 that are well prepared to handle
+ * locking during I/O themselves: either by doing fine grained locking or
+ * by providing lock-free I/O schemes.
+ *
+ * @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.
  *