From: Ian Campbell <ian.campbell@citrix.com>
To: annie li <annie.li@oracle.com>
Cc: <xen-devel@lists.xensource.com>, <netdev@vger.kernel.org>,
<wei.liu2@citrix.com>, <konrad.wilk@oracle.com>, <msw@amazon.com>
Subject: Re: [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb.
Date: Thu, 11 Jul 2013 10:47:58 +0100 [thread overview]
Message-ID: <1373536078.5453.159.camel@hastur.hellion.org.uk> (raw)
In-Reply-To: <51DE6DFD.9080007@oracle.com>
On Thu, 2013-07-11 at 16:34 +0800, annie li wrote:
> On 2013-7-11 16:11, Ian Campbell wrote:
> > On Wed, 2013-07-10 at 17:15 +0800, Annie Li wrote:
> >> +static int netbk_count_slots(struct xenvif *vif, struct sk_buff *skb,
> >> + int *copy_off, unsigned long size,
> >> + unsigned long offset, int *head)
> >> {
> >> - unsigned int count;
> >> - int i, copy_off;
> >> + unsigned long bytes;
> >> + int count = 0;
> >>
> >> - count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> >> + offset &= ~PAGE_MASK;
> >>
> >> - copy_off = skb_headlen(skb) % PAGE_SIZE;
> >> + while (size > 0) {
> >> + BUG_ON(offset >= PAGE_SIZE);
> >> + BUG_ON(*copy_off > MAX_BUFFER_OFFSET);
> >>
> >> - if (skb_shinfo(skb)->gso_size)
> >> - count++;
> >> + bytes = PAGE_SIZE - offset;
> >>
> >> - for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> >> - unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
> >> - unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
> >> - unsigned long bytes;
> >> + if (bytes > size)
> >> + bytes = size;
> >>
> >> - offset &= ~PAGE_MASK;
> >> + if (start_new_rx_buffer(*copy_off, bytes, *head)) {
> >> + count++;
> >> + *copy_off = 0;
> >> + }
> >>
> >> - while (size > 0) {
> >> - BUG_ON(offset >= PAGE_SIZE);
> >> - BUG_ON(copy_off > MAX_BUFFER_OFFSET);
> >> + if (*copy_off + bytes > MAX_BUFFER_OFFSET)
> >> + bytes = MAX_BUFFER_OFFSET - *copy_off;
> >>
> >> - bytes = PAGE_SIZE - offset;
> >> + *copy_off += bytes;
> >>
> >> - if (bytes > size)
> >> - bytes = size;
> >> + offset += bytes;
> >> + size -= bytes;
> >>
> >> - if (start_new_rx_buffer(copy_off, bytes, 0)) {
> >> - count++;
> >> - copy_off = 0;
> >> - }
> >> + /* Next frame */
> >> + if (offset == PAGE_SIZE && size)
> >> + offset = 0;
> >> +
> >> + if (*head)
> >> + count++;
> > This little bit corresponds to the "/* Leave a gap for the GSO
> > descriptor. */" in gop_frag_copy?
>
> No, it does not correspond to this in gop_frag_copy.
So what does it correspond to?
> The code here only
> increase count for the first time. I thought to initialize the count in
> xen_netbk_count_skb_slots with 1 to avoid this. But thinking of the
> extreme case when the header size is zero(not sure whether this case
> could be true), I increase the count here to keep safe in case header
> size is zero.
netfront requires that the first slot always contains some data,
gop_frag_copy will BUG if that's not the case.
>
> There is code correspond to that in gop_frag_copy in
> xen_netbk_count_skb_slots, see following,
>
> + if (skb_shinfo(skb)->gso_size && !vif->gso_prefix)
> + count++;
> (The original code does not have gso_prefix, I added it in this patch
> too based on Wei's suggestion)
OK.
It's be nice if the count and copy variants of this logic could be as
similar as possible but they already differ quite a bit. I have
considered whether we could combine the two by adding "dry-run"
functionality to gop_frag_copy but that seems like it would just ugly up
both versions.
Ian.
next prev parent reply other threads:[~2013-07-11 9:48 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-10 9:15 [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb Annie Li
2013-07-11 2:18 ` David Miller
2013-07-11 2:48 ` [Xen-devel] " annie li
2013-07-11 19:04 ` David Miller
2013-07-11 8:11 ` Ian Campbell
2013-07-11 8:34 ` annie li
2013-07-11 9:47 ` Ian Campbell [this message]
2013-07-11 10:46 ` Annie
2013-07-11 10:59 ` Annie
2013-07-11 11:12 ` [Xen-devel] " Ian Campbell
2013-07-11 13:35 ` annie li
2013-07-11 20:03 ` David Miller
2013-07-11 21:12 ` [Xen-devel] " Wei Liu
2013-07-16 9:00 ` Ian Campbell
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1373536078.5453.159.camel@hastur.hellion.org.uk \
--to=ian.campbell@citrix.com \
--cc=annie.li@oracle.com \
--cc=konrad.wilk@oracle.com \
--cc=msw@amazon.com \
--cc=netdev@vger.kernel.org \
--cc=wei.liu2@citrix.com \
--cc=xen-devel@lists.xensource.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).