linux-kernel - Re: [PATCH RFC 1/3] Drivers: hv: kvp: convert userspace/kernel communication to using char device

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Message-ID: <20150227202715.GF2034@potion.brq.redhat.com>
Date:	Fri, 27 Feb 2015 21:27:15 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	"K. Y. Srinivasan" <kys@...rosoft.com>,
	devel@...uxdriverproject.org,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	linux-kernel@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] Drivers: hv: kvp: convert userspace/kernel
 communication to using char device

2015-02-27 17:14+0100, Vitaly Kuznetsov:
> Re-implement the communication using misc char device. Use ioctl to do
> kernel/userspace version negotiation (doesn't make much sense at this moment
> as we're breaking backwards compatibility but can be used in future).

The main question is whether we want to abolish backward compatibility;
kernel rules are usually against breakages and it's hard to prove that
the bundled daemon is a sole user and gets updated at the same time.
(Note: I'd gladly break anything.)

The ioctl is used too creatively for my liking: as an out-of-band
communication that is required after the main channel has been opened.
It would be simpler to inject the version into first x bytes of the
stream, making a read() after open() mandatory.

(I've only done a high level overview so far.)
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