Re: [PATCH 4/7] hw/scsi/virtio-scsi: Use VIRTIO_SCSI_COMMON() macro

[Kevin Wolf <kwolf@redhat.com>]

Am 17.10.2023 um 16:01 hat Philippe Mathieu-Daudé geschrieben:
> Access QOM parent with the proper QOM VIRTIO_SCSI_COMMON() macro.
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  hw/scsi/virtio-scsi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> index 45b95ea070..fa53f0902c 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -761,7 +761,7 @@ static void virtio_scsi_fail_cmd_req(VirtIOSCSIReq *req)
>  
>  static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq *req)
>  {
> -    VirtIOSCSICommon *vs = &s->parent_obj;
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
>      SCSIDevice *d;
>      int rc;

Why is a dynamic cast more "proper" than a static type-safe access, even
more so in a hot I/O path?

Rich Jones posted a flamegraph the other day that surprised me because
object_class_dynamic_class_assert() and object_dynamic_cast_assert()
were shown to be a big part of scsi_req_new(). In the overall
performance, it's probably dwarved by other issues, but unnecessary
little things can add up, too.

Kevin