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

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Message-ID: <ZqI3ntUR6bfY1kxo@redhat.com>
Date: Thu, 25 Jul 2024 12:31:42 +0100
From: Daniel P. Berrangé <berrange@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: "Michael S. Tsirkin" <mst@...hat.com>,
	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 10:56:05AM +0100, David Woodhouse wrote:
> Hi Michael, thanks for the review!
> 
> On Thu, 2024-07-25 at 01:48 -0400, Michael S. Tsirkin wrote:
> > 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>
> > > ---
> > > QEMU implementation at
> > > https://git.infradead.org/users/dwmw2/qemu.git/shortlog/refs/heads/vmclock
> > > 
> > > Although the ACPI device implemented in QEMU (and some other
> > > hypervisor) stands alone, most of the fields and values herein are
> > > aligned as much as possible with the nascent virtio-rtc specification,
> > > with the intent that a version of the same structure can be
> > > incorporated into that standard.
> > 
> > Do you want to just help complete virtio-rtc then? Would be easier than
> > trying to keep two specs in sync.
> 
> The ACPI version is much more lightweight and doesn't take up a
> valuable PCI slot#. (I know, you can do virtio without PCI but that's
> complex in other ways).

In general it shouldn't have to take up a PCI slot, that's just
a common default policy. virtio-devices only need a dedicated
slot if there's a need to do hotplug/unplug of them. There is a
set of core devices for which hotplug doesn't make sense, which
could all be put as functions in the same slot. ie virtio-rng,
virtio-balloon and virtio-rtc, could all live in one slot.

With regards,
Daniel
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