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Date: Wed, 27 Jan 2016 14:35:58 +0800
From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: "Xie, Huawei" <huawei.xie@intel.com>
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Cc: "dev@dpdk.org" <dev@dpdk.org>, Victor Kaplansky <vkaplans@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [dpdk-dev] [PATCH 4/5] vhost: do not use rte_memcpy for
virtio_hdr copy
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On Wed, Jan 27, 2016 at 06:16:37AM +0000, Xie, Huawei wrote:
> On 1/27/2016 2:02 PM, Yuanhan Liu wrote:
> > On Wed, Jan 27, 2016 at 05:56:56AM +0000, Xie, Huawei wrote:
> >> On 1/27/2016 11:22 AM, Yuanhan Liu wrote:
> >>> On Wed, Jan 27, 2016 at 02:46:39AM +0000, Xie, Huawei wrote:
> >>>> On 12/3/2015 2:03 PM, Yuanhan Liu wrote:
> >>>>> + if (vq->vhost_hlen == sizeof(struct virtio_net_hdr_mrg_rxbuf)) {
> >>>>> + *(struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc_addr = hdr;
> >>>>> + } else {
> >>>>> + *(struct virtio_net_hdr *)(uintptr_t)desc_addr = hdr.hdr;
> >>>>> + }
> >>>> Thanks!
> >>>> We might simplify this further. Just reset the first two fields flags
> >>>> and gso_type.
> >>> What's this "simplification" for? Don't even to say that we will add
> >>> TSO support, which modifies few more files, such as csum_start: reseting
> >>> the first two fields only is wrong here.
> >> I know TSO before commenting, but at least in this implementation and
> >> this specific patch, i guess zeroing two fields are enough.
> >>
> >> What is wrong resetting only two fields?
> > I then have to ask "What is the benifit of resetting only two fields"?
> > If doing so, we have to change it back for TSO. My proposal requires no
> > extra change when adding TSO support.
>
> ? Benefit is we save four unnecessary stores.
Hmm..., the hdr size is 12 bytes at most. I mean, does it really matter,
coping 3 bytes, or coping 12 bytes in a row?
--yliu