Re: [PATCH] system/physmem: mark io_mem_unassigned lockless

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 29/10/25 19:52, Stefan Hajnoczi wrote:
> When the Bus Master bit is disabled in a PCI device's Command Register,
> the device's DMA address space becomes unassigned memory (i.e. the
> io_mem_unassigned MemoryRegion).
> 
> This can lead to deadlocks with IOThreads since io_mem_unassigned
> accesses attempt to acquire the Big QEMU Lock (BQL). For example,
> virtio-pci devices deadlock in virtio_write_config() ->
> virtio_pci_stop_ioeventfd() when waiting for the IOThread while holding
> the BQL. The IOThread is unable to acquire the BQL but the vcpu thread
> won't release the BQL while waiting for the IOThread.
> 
> io_mem_unassigned is trivially thread-safe since it has no state, it
> simply rejects all load/store accesses. Therefore it is safe to enable
> lockless I/O on io_mem_unassigned to eliminate this deadlock.
> 
[...]
> Buglink: https://issues.redhat.com/browse/RHEL-71933
> Reported-by: Peixiu Hou <phou@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>   system/physmem.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/system/physmem.c b/system/physmem.c
> index a340ca3e61..1dc2b46e12 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -3011,6 +3011,9 @@ static void io_mem_init(void)
>   {
>       memory_region_init_io(&io_mem_unassigned, NULL, &unassigned_mem_ops, NULL,
>                             NULL, UINT64_MAX);
> +
> +    /* Trivially thread-safe since memory accesses are rejected */

Maybe duplicate the comment in unassigned_mem_ops[]. Regardless,

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

> +    memory_region_enable_lockless_io(&io_mem_unassigned);
>   }