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Message-ID: <2b581879-f449-4ba3-8fbd-397d0f23ae15@sberdevices.ru>
Date: Tue, 1 Aug 2023 16:28:53 +0300
From: Arseniy Krasnov <avkrasnov@...rdevices.ru>
To: Paolo Abeni <pabeni@...hat.com>,
Stefan Hajnoczi <stefanha@...hat.com>,
Stefano Garzarella <sgarzare@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
"Michael S. Tsirkin" <mst@...hat.com>,
Jason Wang <jasowang@...hat.com>,
Bobby Eshleman <bobby.eshleman@...edance.com>
CC: <kvm@...r.kernel.org>, <virtualization@...ts.linux-foundation.org>,
<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<kernel@...rdevices.ru>, <oxffffaa@...il.com>
Subject: Re: [PATCH net-next v5 1/4] vsock/virtio/vhost: read data from
non-linear skb
On 01.08.2023 16:11, Paolo Abeni wrote:
> On Sun, 2023-07-30 at 11:59 +0300, Arseniy Krasnov wrote:
>> This is preparation patch for MSG_ZEROCOPY support. It adds handling of
>> non-linear skbs by replacing direct calls of 'memcpy_to_msg()' with
>> 'skb_copy_datagram_iter()'. Main advantage of the second one is that it
>> can handle paged part of the skb by using 'kmap()' on each page, but if
>> there are no pages in the skb, it behaves like simple copying to iov
>> iterator. This patch also adds new field to the control block of skb -
>> this value shows current offset in the skb to read next portion of data
>> (it doesn't matter linear it or not). Idea behind this field is that
>> 'skb_copy_datagram_iter()' handles both types of skb internally - it
>> just needs an offset from which to copy data from the given skb. This
>> offset is incremented on each read from skb. This approach allows to
>> avoid special handling of non-linear skbs:
>> 1) We can't call 'skb_pull()' on it, because it updates 'data' pointer.
>> 2) We need to update 'data_len' also on each read from this skb.
>
> It looks like the above sentence is a left-over from previous version
> as, as this patch does not touch data_len. And I think it contradicts
> the previous one, so it's a bit confusing.
Yes, seems I need to rephrase it in the next version. I meant that with
approach introduced in this patch we don't need to check that skb is
linear of non-linear after reading data from it. Because otherwise:
1) In case of linear skb we will need to call 'skb_pull()' after reading
data, to update 'data' pointer.
2) In case of non-linear skb we will need to update 'data_len' field after
reading data, as this field shows amount of data in fragged part.
>
>> Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>> ---
>> Changelog:
>> v5(big patchset) -> v1:
>> * Merge 'virtio_transport_common.c' and 'vhost/vsock.c' patches into
>> this single patch.
>> * Commit message update: grammar fix and remark that this patch is
>> MSG_ZEROCOPY preparation.
>> * Use 'min_t()' instead of comparison using '<>' operators.
>> v1 -> v2:
>> * R-b tag added.
>> v3 -> v4:
>> * R-b tag removed due to rebase:
>> * Part for 'virtio_transport_stream_do_peek()' is changed.
>> * Part for 'virtio_transport_seqpacket_do_peek()' is added.
>> * Comments about sleep in 'memcpy_to_msg()' now describe sleep in
>> 'skb_copy_datagram_iter()'.
>>
>> drivers/vhost/vsock.c | 14 +++++++----
>> include/linux/virtio_vsock.h | 1 +
>> net/vmw_vsock/virtio_transport_common.c | 32 +++++++++++++++----------
>> 3 files changed, 29 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>> index 817d377a3f36..8c917be32b5d 100644
>> --- a/drivers/vhost/vsock.c
>> +++ b/drivers/vhost/vsock.c
>> @@ -114,6 +114,7 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
>> struct sk_buff *skb;
>> unsigned out, in;
>> size_t nbytes;
>> + u32 frag_off;
>
> IMHO 'offset' would be a better name for both the variable and the CB
> field, as it can points both inside the skb frags, linear part or frag
> list.
Ack
>
> Otherwise LGTM, thanks!
>
> Paolo
>
Thanks, Arseniy
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