Re: [PATCH v3 01/10] memory: reintroduce BQL-free fine-grained PIO/MMIO

[David Hildenbrand <david@redhat.com>]

On 08.08.25 14:01, Igor Mammedov wrote:
> 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>
> ---
> v3:
>    add comment for 'mr->disable_reentrancy_guard = true'
>      Peter Xu <peterx@redhat.com>
> ---
>   include/system/memory.h | 10 ++++++++++
>   system/memory.c         | 15 +++++++++++++++
>   system/physmem.c        |  2 +-
>   3 files changed, 26 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);

Is this safe to use on any IO region, or could actually something break 
when mis-used? In case it's the latter, I assume we would want to 
carefully document under which scenarios this is safe to use.

-- 
Cheers,

David / dhildenb