Re: [Qemu-devel] [PATCH v2 2/2] virtio-scsi: Optimize virtio_scsi_init_req

[Paolo Bonzini <pbonzini@redhat.com>]

Il 15/09/2014 07:23, Fam Zheng ha scritto:
>      SCSIRequest *sreq;
>      size_t resp_size;
>      enum SCSIXferMode mode;
> -    QEMUIOVector resp_iov;
>      union {
>          VirtIOSCSICmdResp     cmd;
>          VirtIOSCSICtrlTMFResp tmf;
> @@ -68,23 +75,27 @@ static inline SCSIDevice *virtio_scsi_device_find(VirtIOSCSI *s, uint8_t *lun)
>  static VirtIOSCSIReq *virtio_scsi_init_req(VirtIOSCSI *s, VirtQueue *vq)
>  {
>      VirtIOSCSIReq *req;
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
> -
> -    req = g_malloc0(sizeof(*req) + vs->cdb_size);
> +    VirtIOSCSICommon *vs = (VirtIOSCSICommon *)s;
> +    const size_t zero_skip = offsetof(VirtIOSCSIReq, elem)
> +                             + sizeof(VirtQueueElement);
>  
> +    req = g_slice_alloc(sizeof(*req) + vs->cdb_size);

Looks good, but why do you need to zero the union?  You only need to
zero sreq, resp_size and mode, don't you (and at this point, memset
becomes superfluous)?

Paolo