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

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Message-ID: <20240726015613-mutt-send-email-mst@kernel.org>
Date: Fri, 26 Jul 2024 02:06:10 -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 11:20:56PM +0100, David Woodhouse wrote:
> We're rolling out the AMZNVCLK device for internal use cases, and plan
> to add it in public instances some time later.

Let's be real. If amazon does something in its own hypervisor, and the
only way to use that is to expose the interface to userspace, there is
very little the linux community can do.  Moreover, userspace will be
written to this ABI, and be locked in to the specific hypervisor. It
might be a win for amazon short term but long term you will want to
extend things and it will be a mess.

So I feel you have chosen ACPI badly.  It just does not have the APIs
that you need. Virtio does, and would not create a userpspace lock-in
to a specific hypervisor. It's not really virtio specific either,
you can write a bare pci device with a BAR and a bunch of msix
vectors and it will get you the same effect.

-- 
MST