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Message-ID: <Y/N7+IJ+gzvP0IEf@nanopsycho>
Date: Mon, 20 Feb 2023 14:56:08 +0100
From: Jiri Pirko <jiri@...nulli.us>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
pabeni@...hat.com, edumazet@...gle.com, jasowang@...hat.com,
virtualization@...ts.linux-foundation.org,
Vitaly Mireyno <vmireyno@...vell.com>
Subject: Re: [patch net-next] net: virtio_net: implement exact header length
guest feature
Mon, Feb 20, 2023 at 01:55:33PM CET, mst@...hat.com wrote:
>On Mon, Feb 20, 2023 at 09:35:00AM +0100, Jiri Pirko wrote:
>> Fri, Feb 17, 2023 at 02:47:36PM CET, mst@...hat.com wrote:
>> >On Fri, Feb 17, 2023 at 01:53:55PM +0100, Jiri Pirko wrote:
>> >> Fri, Feb 17, 2023 at 01:22:01PM CET, mst@...hat.com wrote:
>> >> >On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:
>> >> >> From: Jiri Pirko <jiri@...dia.com>
>> >> >>
>> >> >> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
>> >> >>
>> >> >> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
>> >> >> set implicates that the driver provides the exact size of the header.
>> >> >>
>> >> >> The driver already complies to fill the correct value. Introduce the
>> >> >> feature and advertise it.
>> >> >>
>> >> >> Signed-off-by: Jiri Pirko <jiri@...dia.com>
>> >> >
>> >> >Could you add a bit of motivation just for the record?
>> >> >Does this improve performance for some card? By how much?
>> >> >Expected to help some future card?
>> >>
>> >> I can get that info, but isn't that rather something to be appended to
>> >> the virtio-spec patch? I mean, the feature is there, this is just
>> >> implementing it in one driver.
>> >
>> >It is more like using it in the driver. It's not like we have to use
>> >everything - it could be useful for e.g. dpdk but not linux.
>> >Implementing it in the Linux driver has support costs - for example what
>> >if there's a bug and sometimes the length is incorrect?
>> >We'll be breaking things.
>>
>> I understand. To my understanding this feature just fixes the original
>> ambiguity in the virtio spec.
>>
>> Quoting the original virtio spec:
>> "hdr_len is a hint to the device as to how much of the header needs to
>> be kept to copy into each packet"
>>
>> "a hint" might not be clear for the reader what does it mean, if it is
>> "maybe like that" of "exactly like that". This feature just makes it
>> crystal clear.
>>
>> If you look at the tap implementation, it uses hdr_len to alloc
>> skb linear part. No hint, it counts with the provided value.
>> So if the driver is currently not precise, it breaks tap.
>
>Well that's only for gso though right?
Yep.
>And making it bigger than necessary works fine ...
Well yeah. But tap does not do that, does it? it uses hdr_len directly.
>
>> I will add this to the patch description and send v2.
>>
>
>I feel this does not answer the question yet, or maybe I am being dense.
>My point was not about making hdr_len precise. My point was that we are
>making a change here for no apparent reason. I am guessing you are not
>doing it for fun - so why? Is there a device with this feature bit
>you are aware of?
Afaik real hw which does emulation of virtio_net would benefit from
that, our hw including.
>
>
>
>>
>> >
>> >The patch was submitted by Marvell but they never bothered with
>> >using it in Linux. I guess they are using it for something else?
>> >CC Vitaly who put this in.
>> >
>> >>
>> >> >
>> >> >thanks!
>> >> >
>> >> >
>> >> >> ---
>> >> >> drivers/net/virtio_net.c | 6 ++++--
>> >> >> include/uapi/linux/virtio_net.h | 1 +
>> >> >> 2 files changed, 5 insertions(+), 2 deletions(-)
>> >> >>
>> >> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> >> >> index fb5e68ed3ec2..e85b03988733 100644
>> >> >> --- a/drivers/net/virtio_net.c
>> >> >> +++ b/drivers/net/virtio_net.c
>> >> >> @@ -62,7 +62,8 @@ static const unsigned long guest_offloads[] = {
>> >> >> VIRTIO_NET_F_GUEST_UFO,
>> >> >> VIRTIO_NET_F_GUEST_CSUM,
>> >> >> VIRTIO_NET_F_GUEST_USO4,
>> >> >> - VIRTIO_NET_F_GUEST_USO6
>> >> >> + VIRTIO_NET_F_GUEST_USO6,
>> >> >> + VIRTIO_NET_F_GUEST_HDRLEN
>> >> >> };
>> >> >>
>> >> >> #define GUEST_OFFLOAD_GRO_HW_MASK ((1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
>> >> >> @@ -4213,7 +4214,8 @@ static struct virtio_device_id id_table[] = {
>> >> >> VIRTIO_NET_F_CTRL_MAC_ADDR, \
>> >> >> VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
>> >> >> VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
>> >> >> - VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL
>> >> >> + VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL, \
>> >> >> + VIRTIO_NET_F_GUEST_HDRLEN
>> >> >>
>> >> >> static unsigned int features[] = {
>> >> >> VIRTNET_FEATURES,
>> >> >> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
>> >> >> index b4062bed186a..12c1c9699935 100644
>> >> >> --- a/include/uapi/linux/virtio_net.h
>> >> >> +++ b/include/uapi/linux/virtio_net.h
>> >> >> @@ -61,6 +61,7 @@
>> >> >> #define VIRTIO_NET_F_GUEST_USO6 55 /* Guest can handle USOv6 in. */
>> >> >> #define VIRTIO_NET_F_HOST_USO 56 /* Host can handle USO in. */
>> >> >> #define VIRTIO_NET_F_HASH_REPORT 57 /* Supports hash report */
>> >> >> +#define VIRTIO_NET_F_GUEST_HDRLEN 59 /* Guest provides the exact hdr_len value. */
>> >> >> #define VIRTIO_NET_F_RSS 60 /* Supports RSS RX steering */
>> >> >> #define VIRTIO_NET_F_RSC_EXT 61 /* extended coalescing info */
>> >> >> #define VIRTIO_NET_F_STANDBY 62 /* Act as standby for another device
>> >> >> --
>> >> >> 2.39.0
>> >> >
>> >
>
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