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Subject: RE: [EXT] Re: [virtio-comment] [PATCH] virtio-net: Add an optional device control over the receive buffers length
>-----Original Message-----
>From: Michael S. Tsirkin <mst@redhat.com>
>Sent: Monday, 20 January, 2020 18:25
>To: Vitaly Mireyno <vmireyno@marvell.com>
>Cc: virtio-comment@lists.oasis-open.org; Jason Wang
><jasowang@redhat.com>
>Subject: [EXT] Re: [virtio-comment] [PATCH] virtio-net: Add an optional device
>control over the receive buffers length
>
>External Email
>
>----------------------------------------------------------------------
>On Mon, Dec 30, 2019 at 01:59:17PM +0000, Vitaly Mireyno wrote:
>> This patch gives devices some level of control over the receive buffers
>length.
>> The driver declares the minimum receive buffer length, and the device
>requests max/min buffer length ratio.
>> This is a follow-up to the "[PATCH] virtio-net: Add equal-sized
>> receive buffers feature" discussion:
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.oasis-2Dope
>> n.org_archives_virtio-
>2Dcomment_201912_msg00007.html&d=DwIFAg&c=nKjWec
>>
>2b6R0mOyPaz7xtfQ&r=lDHJ2FW52oJ3lqqsArgFRdcevq01tbLQAw4A_NO7xgI&
>m=jrQgZ
>> -
>iN3QnvFqxQQkoPTmztJRTNCXTF2dz68TEV0cA&s=pDtISMJpw1N9ohot9fluo1N
>j1X2s1
>> J_sEVJGg2uwkZk&e=
>>
>>
>> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
>> ---
>> content.tex | 41 +++++++++++++++++++++++++++++++++++++++--
>> 1 file changed, 39 insertions(+), 2 deletions(-)
>>
>> diff --git a/content.tex b/content.tex index d68cfaf..0a4cfba 100644
>> --- a/content.tex
>> +++ b/content.tex
>> @@ -2815,6 +2815,13 @@ \subsection{Feature bits}\label{sec:Device
>> Types / Network Device / Feature bits
>\item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through
>control
>> channel.
>>
>> +\item[VIRTIO_NET_F_RXBUF_LEN_RATIO(57)] Device requests to limit the
>> + maximum/minimum receive buffers length ratio.
>> +
>> +\item[VIRTIO_NET_F_RXBUF_MIN_LEN(58)] Device requests to know the
>minimum
>> + receive buffers length. Driver declares the minimum receive buffers
>> + length.
>> +
>> \item[VIRTIO_NET_F_GUEST_HDRLEN(59)] Driver can provide the exact
>\field{hdr_len}
>> value. Device benefits from knowing the exact header length.
>>
>
>So I am wondering, when we are switching from regular skbs to XDP, all
>buffers become 4K. When we switch back, we get variability again.
>All the while some old buffers can be outstanding.
>
>How will we handle this?
>
The feature limits the flexibility of the driver to set the descriptors length.
It doesn't limit the actual buffer allocation size. So if XDP requires a larger headroom, I don't see a problem. Please excuse my ignorance, but what do you mean by "all buffers become 4K"?
>
>> @@ -2861,8 +2868,8 @@ \subsubsection{Legacy Interface: Feature
>> bits}\label{sec:Device Types / Network \subsection{Device
>> configuration layout}\label{sec:Device Types / Network Device / Device
>> configuration layout} \label{sec:Device Types / Block Device /
>> Feature bits / Device configuration layout}
>>
>> -Three driver-read-only configuration fields are currently defined.
>> The \field{mac} address field -always exists (though is only valid if
>> VIRTIO_NET_F_MAC is set), and
>> +The driver-read-only \field{mac} address field always exists (though
>> +is only valid if VIRTIO_NET_F_MAC is set), and
>> \field{status} only exists if VIRTIO_NET_F_STATUS is set. Two
>> read-only bits (for the driver) are currently defined for the status field:
>> VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
>> @@ -2882,12 +2889,22 @@ \subsection{Device configuration
>> layout}\label{sec:Device Types / Network Device VIRTIO_NET_F_MTU is
>> set. This field specifies the maximum MTU for the driver to use.
>>
>> +The device-read-only field \field{min_rx_buf_len} only exists if
>> +VIRTIO_NET_F_RXBUF_MIN_LEN is set. This field specifies the minimum
>> +receive buffers length.
>> +
>> +The driver-read-only field \field{rx_buf_len_ratio} only exists if
>> +VIRTIO_NET_F_RXBUF_LEN_RATIO is set. This field specifies the
>> +maximum/minimum receive buffers length ratio.
>> +
>> \begin{lstlisting}
>> struct virtio_net_config {
>> u8 mac[6];
>> le16 status;
>> le16 max_virtqueue_pairs;
>> le16 mtu;
>> + le32 min_rx_buf_len;
>> + le16 rx_buf_len_ratio;
>> };
>> \end{lstlisting}
>>
>> @@ -2916,6 +2933,15 @@ \subsection{Device configuration
>> layout}\label{sec:Device Types / Network Device If the driver
>> negotiates the VIRTIO_NET_F_STANDBY feature, the device MAY act as a
>standby device for a primary device with the same MAC address.
>>
>> +A driver SHOULD accept the VIRTIO_NET_F_RXBUF_MIN_LEN feature if
>offered.
>> +
>> +If VIRTIO_NET_F_RXBUF_MIN_LEN feature has been negotiated, the
>driver
>> +MUST set \field{min_rx_buf_len}.
>> +
>> +A driver MUST NOT modify \field{min_rx_buf_len} once it has been set.
>> +
>> +A driver SHOULD accept the VIRTIO_NET_F_RXBUF_LEN_RATIO feature if
>offered.
>> +
>> \drivernormative{\subsubsection}{Device configuration layout}{Device
>> Types / Network Device / Device configuration layout}
>>
>> A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it.
>> @@ -3281,6 +3307,17 @@ \subsubsection{Setting Up Receive
>> Buffers}\label{sec:Device Types / Network Devi If VIRTIO_NET_F_MQ is
>> negotiated, each of receiveq1\ldots receiveqN that will be used SHOULD be
>populated with receive buffers.
>>
>> +If VIRTIO_NET_F_RXBUF_MIN_LEN feature has been negotiated, the
>driver
>> +MUST initialize all receive virtqueue descriptors \field{len} field
>> +with the value greater than or equal to the value configured in the
>> +\field{min_rx_buf_len} device configuration field, and allocate
>> +receive buffers accordingly.
>> +
>> +If VIRTIO_NET_F_RXBUF_LEN_RATIO feature has been negotiated, the
>> +driver MUST initialize all receive virtqueue descriptors \field{len}
>> +field with the value less than or equal to (\field{min_rx_buf_len} *
>> +\field{rx_buf_len_ratio}), and allocate receive buffers accordingly.
>> +
>> \devicenormative{\paragraph}{Setting Up Receive Buffers}{Device Types
>> / Network Device / Device Operation / Setting Up Receive Buffers}
>>
>> The device MUST set \field{num_buffers} to the number of descriptors
>> used to
>> --
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