Re: [virtio-comment] [PATCH v4 1/1] virtio-net: Define per-packet hash reporting feature

[Yuri Benditovich <ybendito@redhat.com>]

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----- Original Message -----

> From: "Jason Wang" <jasowang@redhat.com>
> To: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: "Yuri Benditovich" <yuri.benditovich@daynix.com>,
> virtio-comment@lists.oasis-open.org
> Sent: Thursday, February 20, 2020 8:38:50 AM
> Subject: Re: [virtio-comment] [PATCH v4 1/1] virtio-net: Define per-packet
> hash reporting feature

> On 2020/2/19 下午10:23, Michael S. Tsirkin wrote:
> > On Wed, Feb 19, 2020 at 08:27:10PM +0800, Jason Wang wrote:
> >> On 2020/2/19 下午3:53, Yuri Benditovich wrote:
> >>> The device MUST set \field{rss_max_indirection_table_length} to at least
> >>> 128, if it offers
> >>> VIRTIO_NET_F_RSS.
> >>> @@ -3195,6 +3180,8 @@ \subsection{Device Operation}\label{sec:Device
> >>> Types / Network Device / Device O
> >>> le16 csum_start;
> >>> le16 csum_offset;
> >>> le16 num_buffers;
> >>> + le32 hash_value; (Only if VIRTIO_NET_F_HASH_REPORT negotiated)
> >>> + le16 hash_type; (Only if VIRTIO_NET_F_HASH_REPORT negotiated)
> >>> };
> >>> \end{lstlisting}
> >> A question here:
> >>
> >> Consider we introduce VIRTIO_NET_F_FEATURE_INFORMATION in the future:
> >>
> >> le32 hash_type; // VIRTIO_NET_F_HASH_REPORT
> >> le32 feature_information; // VIRTIO_NET_F_FEATURE_INFORMATION
> >>
> >> What happens if HASH_REPORT is not negotiated, I believe we expect a
> >> stable
> >> ABI(offset) here for feature_information?
> >>
> >> Thanks
> >>
> > We'll have to decide at that point ... any better ideas?

> Not sure but something that is self descriptive? (which could be an
> overkill for fields that only need few bytes).

The problem is that the driver typically wants to know the header size from the beginning to configure SG table. 
So any self-descriptive layout seems good but in practice not so usable, IMO. 

> Thanks

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