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Subject: Re: [MASSMAIL KLMS] Re: [virtio-comment] Re: [MASSMAIL KLMS] Re: [virtio-comment] Re: [MASSMAIL KLMS] Re: [virtio-comment] [RFC PATCH v4 2/2] virtio-vsock: SOCK_SEQPACKET description
On 09.04.2021 17:27, Stefano Garzarella wrote:
> On Sat, Apr 03, 2021 at 12:45:54PM +0300, Arseny Krasnov wrote:
>> On 31.03.2021 17:48, Stefan Hajnoczi wrote:
>>> On Tue, Mar 30, 2021 at 05:24:19PM +0300, Arseny Krasnov wrote:
>>>> On 30.03.2021 16:57, Stefan Hajnoczi wrote:
>>>>> On Tue, Mar 30, 2021 at 12:50:06PM +0300, Arseny Krasnov wrote:
>>>>>> On 30.03.2021 11:55, Stefan Hajnoczi wrote:
>>>>>>> On Tue, Mar 30, 2021 at 09:15:39AM +0300, Arseny Krasnov wrote:
>>>>>>>> On 30.03.2021 00:28, Stefano Garzarella wrote:
>>>>>>>>> On Mon, Mar 29, 2021 at 08:33:27PM +0300, Arseny Krasnov wrote:
>>>>>>>>>> On 29.03.2021 19:11, Stefan Hajnoczi wrote:
>>>>>>>>>>> On Fri, Mar 26, 2021 at 12:02:50PM +0300, Arseny Krasnov wrote:
>>>>>>>>>>>> @@ -98,6 +102,10 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op
>>>>>>>>>>>> #define VIRTIO_VSOCK_OP_CREDIT_UPDATE 6
>>>>>>>>>>>> /* Request the peer to send the credit info to us */
>>>>>>>>>>>> #define VIRTIO_VSOCK_OP_CREDIT_REQUEST 7
>>>>>>>>>>>> +/* Message begin for SOCK_SEQPACKET */
>>>>>>>>>>>> +#define VIRTIO_VSOCK_OP_SEQ_BEGIN 8
>>>>>>>>>>>> +/* Message end for SOCK_SEQPACKET */
>>>>>>>>>>>> +#define VIRTIO_VSOCK_OP_SEQ_END 9
>>>>>>>>>>> The struct virtio_vsock_hdr->flags field is le32 and currently unused.
>>>>>>>>>>> Could 24 bits be used for a unique message id and 8 bits for flags? 1
>>>>>>>>>>> flag bit could be used for end-of-message and the remaining 7 bits could
>>>>>>>>>>> be reserved. That way SEQ_BEGIN and SEQ_END are not necessary.
>>>>>>>>>>> Pressure
>>>>>>>>>>> on the virtqueue would be reduced and performance should be comparable
>>>>>>>>>>> to SOCK_STREAM.
>>>>>>>>>> Well, my first versions of SOCK_SEQPACKET implementation, worked
>>>>>>>>>> something like this: i used flags field of header as length of whole
>>>>>>>>>> message. I discussed it with Stefano Garzarella, and he told that it
>>>>>>>>>> will
>>>>>>>>>> be better to use special "header" in packet's payload, to keep some
>>>>>>>>>> SOCK_SEQPACKET specific data, instead of reusing packet's header
>>>>>>>>>> fields.
>>>>>>>>> IIRC in the first implementation SEQ_BEGIN was an empty message and we
>>>>>>>>> didn't added the msg_id yet. So since we needed to carry both id and
>>>>>>>>> total length, I suggested to use the payload to put these extra
>>>>>>>>> information.
>>>>>>>>>
>>>>>>>>> IIUC what Stefan is suggesting is a bit different and I think it should
>>>>>>>>> be cool to implement: we can remove the boundary packets, use only 8
>>>>>>>>> bits for the flags, and add a new field to reuse the 24 unused bits,
>>>>>>>>> maybe also 16 bits would be enough.
>>>>>>>>> At that point we will only use the EOR flag to know the last packet.
>>>>>>>>>
>>>>>>>>> The main difference will be that the receiver will know the total size
>>>>>>>>> only when the last packet is received.
>>>>>>>>>
>>>>>>>>> Do you see any issue on that approach?
>>>>>>>> It will work, except we can't check that any packet of message,
>>>>>>>>
>>>>>>>> except last(with EOR bit) was dropped, since receiver don't know
>>>>>>>>
>>>>>>>> real length of message. If it is ok, then i can implement it.
>>>>>>> The credit mechanism ensures that packets are not dropped, so it's not
>>>>>>> necessary to check for this condition.
>>>>>>>
>>>>>>> By the way, is a unique message ID needed? My understanding is:
>>>>>>>
>>>>>>> If two messages are being sent on a socket at the same time either their
>>>>>>> order is serialized (whichever message began first) or it is undefined
>>>>>>> (whichever message completes first).
>>>>>> If we are talking about case, when two threads writes to one socket at the same time,
>>>>>>
>>>>>> in Linux it is possible that two message will interleave(for vsock). But as i know, for example
>>>>>>
>>>>>> when TCP socket is used, both 'write()' calls will be serialized. May be it is bug of vsock: when
>>>>>>
>>>>>> first writer goes out of space, it will sleep. Then second writer tries to send something, but
>>>>>>
>>>>>> as free space is over, it will sleep too. Then, credit update is received from peer. Both sender's
>>>>>>
>>>>>> will be woken up, but sender which grab socket lock first will continue to send it's message.
>>>>>>
>>>>>> So may be we can add something like semaphore to new/vmw_vsock/af_vsock.c which will
>>>>>>
>>>>>> serialize two 'write()' calls: second sender enters 'write()' path, only when first left this path.
>>>>>>
>>>>>> My implementation doesn't care about that, because i wanted to add semaphore later, by another
>>>>>>
>>>>>> patch.
>>>>> Yes, that is definitely an issue that the driver needs to take care of
>>>>> if we don't have unique message IDs. Thanks for explaining!
>>>> So may I include patch with serializer to next version of my patchset?
>>> Sounds good!
>> There is small problem with approach, when we remove SEQ_BEGIN/SEQ_END
>>
>> bounds of messages in flags field(EOR bit ) of packet header: consider case, when vhost sends data
>>
>> to guest(let it be 64kb). In vhost vsock driver, big buffer of packet(for example 64kb) will
>>
>> be splitted to small buffers, to fit guests's virtio rx descriptors(in current implementation of
>>
>> Linux it is 4kb). All fields of new headers in rx queue will be copied from fields of header
>>
>> of big packet(except len field). Thus we get 16 headers with EOR bit set.
>> set.
>>
>>
>> May be it is possible to:
>>
>> 1) Handle such bit in vhost driver(set EOR bit in flags only for last small buffer of guests rx queue)
>>
>> OR
>>
>> 2) I can remove SEQ_BEGIN, msg_len and msg_id. But keep SEQ_END op. This will be special
>>
>> packet which marks end of message without any payload. In this case, such packet will be
>>
>> processed by vhost "as is".
>>
>>
>> What do You think?
>>
> IMHO option 1 is the best and should not be too complicated to
> implement.
>
> Do you see a specific issue?
I think so, i've already implemented 1). I'll send patchset in next few days
along with spec patch
Thank You
>
> Thanks,
> Stefano
>
>
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