Message-ID: <679ce219-d6f8-88a3-2aef-9de506b624ad@uni-rostock.de>
Date:   Thu, 2 Jun 2022 10:32:57 +0200
From:   Benjamin Beichler <Benjamin.Beichler@...-rostock.de>
To:     Johannes Berg <johannes@...solutions.net>, <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>
CC:     <linux-um@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] um: read multiple msg from virtio slave request fd

Am 01.06.2022 um 19:13 schrieb Johannes Berg:
> On Wed, 2022-06-01 at 15:37 +0000, Benjamin Beichler wrote:
>
> Hmm. How did you run into this? Why would a device send many messages
> and not wait for ACK, but the kernel side actually waits for ACK? What
> would the use case for that be? Seems a bit odd, if both wait for ACK
> there shouldn't be an issue?
>
> Anyway, I guess I don't mind fixing this regardless of whether I see a
> use case where it could happen :-)

Here is my (admittedly maybe odd) case:

I want to use hwsim over virtio with UML but without time travel (as a 
precursor for a later version with TT)

I modified wmediumd to strip out the scheduler dependency and wrote a 
very simple simulation, which simply forwards all frames to all radios. 
Furthermore, I use the usfstl "loop" as main driver to poll all fds 
without time travel. This leads to the situation, that when a msg is put 
on the RX-ring of an uml instance, which also sent concurrently a kick 
(e.g., also trying to send a frame), this creates a deadlock. In the 
original wmediumd this was handled by kind of a hack, calling the loop 
implementation to answer the kick, before sending out a call msg. I need 
to rip out this workaround, because without the usfstl scheduler, it 
created a deep recursion of the loop implementation with additional 
problems.

Nonetheless, even if this would be kind of an optimization: it is 
feasible to wait for the ACK asynchronously, as long as it arrives in 
the same point of simulation time (or as you called it calender). For 
many uml-instances, which could easily run in parallel, this allows an 
easier implementation (at least in my planning :-) ). Of course, it 
would be hard to distinguish, which call-request was acked, but at the 
end wmediumd (and I also plan to do so) simply aborts when the ack is 
negative, so the actual corresponding call is not that important to know.

>
> This code changed a bit, you should rebase onto the uml tree's for-next
> branch.
My bad, I was not expecting someone to change something it that corner 
of the kernel, I only used the latest master and not the next. I will 
redo the patch with ease.
>
>> +	while (1) {
>> +		if (vhost_user_recv_req(vu_dev, &msg.msg,
>> +					sizeof(msg.msg.payload)
>> +					+ sizeof(msg.extra_payload)))
> prefer to keep the + on the previous line.
>
It slightly hits the 80 column restriction, but I would also prefer not 
to break. :-D
> That said, my attempt at rebasing this made it all fail completely,
> maybe you have better luck :)
>
> johannes
>
kind regards

Benjamin

-- 
M.Sc. Benjamin Beichler

Universität Rostock, Fakultät für Informatik und Elektrotechnik
Institut für Angewandte Mikroelektronik und Datentechnik

University of Rostock, Department of CS and EE
Institute of Applied Microelectronics and CE

Richard-Wagner-Straße 31
18119 Rostock
Deutschland/Germany

phone: +49 (0) 381 498 - 7278
email:Benjamin.Beichler@...-rostock.de
www:http://www.imd.uni-rostock.de/

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