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Re: [Qemu-devel] [PATCH v2 2/2] virtio-scsi: Optimize virtio_scsi_init_r
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH v2 2/2] virtio-scsi: Optimize virtio_scsi_init_req |
Date: |
Tue, 16 Sep 2014 15:16:11 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Mon, 09/15 12:17, Paolo Bonzini wrote:
> 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)?
>
The structures in unions are not zeroed by caller, also leaving them breaks
virtio-scsi in my test.
FWIW, I will remove the "req->sreq = NULL;" two lines below in v3. At this
point tuning these small fields are subtle optimization compared to the arrays,
I say let's just simply keep the memset so that adding more fields in the
future are also safe.
Fam