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

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Message-ID: <20240725015120-mutt-send-email-mst@kernel.org>
Date: Thu, 25 Jul 2024 01:54:10 -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 Wed, Jul 24, 2024 at 06:16:37PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The vmclock "device" provides a shared memory region with precision clock
> information. By using shared memory, it is safe across Live Migration.
> 
> Like the KVM PTP clock, this can convert TSC-based cross timestamps into
> KVM clock values. Unlike the KVM PTP clock, it does so only when such is
> actually helpful.
> 
> The memory region of the device is also exposed to userspace so it can be
> read or memory mapped by application which need reliable notification of
> clock disruptions.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>

one other thing worth mentioning is that this design can't work
with confidential computing setups. By comparison, mapping e.g. a
range in a PCI BAR would work for these setups.
Is there a reason this functionality is not interesting for
confidential VMs?

-- 
MST