linux-kernel - Re: [PATCH] ptp: Add vDSO-style vmclock support

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Message-ID: <20240725100351-mutt-send-email-mst@kernel.org>
Date: Thu, 25 Jul 2024 10:11:18 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Richard Cochran <richardcochran@...il.com>,
	Peter Hilber <peter.hilber@...nsynergy.com>,
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org,
	"Ridoux, Julien" <ridouxj@...zon.com>, virtio-dev@...ts.linux.dev,
	"Luu, Ryan" <rluu@...zon.com>,
	"Chashper, David" <chashper@...zon.com>,
	"Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>,
	"Christopher S . Hall" <christopher.s.hall@...el.com>,
	Jason Wang <jasowang@...hat.com>, John Stultz <jstultz@...gle.com>,
	netdev@...r.kernel.org, Stephen Boyd <sboyd@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	qemu-devel <qemu-devel@...gnu.org>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH] ptp: Add vDSO-style vmclock support

On Thu, Jul 25, 2024 at 02:50:50PM +0100, David Woodhouse wrote:
> Even if the virtio-rtc specification were official today, and I was
> able to expose it via PCI, I probably wouldn't do it that way. There's
> just far more in virtio-rtc than we need; the simple shared memory
> region is perfectly sufficient for most needs, and especially ours.

I can't stop amazon from shipping whatever in its hypervisor,
I'd just like to understand this better, if there is a use-case
not addressed here then we can change virtio to address it.

The rtc driver patch posted is 900 lines, yours is 700 lines, does not
look like a big difference.  As for using a memory region, this is
valid, but maybe rtc should be changed to do exactly that?
E.g. we can easily add a capability describing such a region.
or put it in device config space.

I mean yes, we can build a new transport for each specific need but in
the end we'll get a ton of interfaces with unclear compatibility
requirements.  If effort is instead spent improving common interfaces,
we get consistency and everyone benefits. That's why I'm trying to
understand the need here.

-- 
MST