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

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Message-ID: <20240726090326-mutt-send-email-mst@kernel.org>
Date: Fri, 26 Jul 2024 09:04:38 -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 Fri, Jul 26, 2024 at 02:00:25PM +0100, David Woodhouse wrote:
> On Fri, 2024-07-26 at 08:52 -0400, Michael S. Tsirkin wrote:
> > On Fri, Jul 26, 2024 at 09:35:51AM +0100, David Woodhouse wrote:
> > > But for this use case, we only need a memory region that the hypervisor
> > > can update. We don't need any of that complexity of gratuitously
> > > interrupting all the vCPUs just to ensure that none of them can be
> > > running userspace while one of them does an update for itself,
> > > potentially translating from one ABI to another. The hypervisor can
> > > just update the user-visible memory in place.
> > 
> > Looks like then your userspace is hypervisor specific, and that's a
> > problem because it's a one way street - there is no way for hypervisor
> > to know what does userspace need, so no way for hypervisor to know which
> > information to provide. No real way to fix bugs.
> 
> It's not hypervisor specific, but you're right that as it stands there
> is no negotiation of what userspace wants. So the hypervisor provides
> what it feels it can provide without significant overhead (which may or
> may not include the precise timekeeping, as discussed, but should
> always include the disruption signal which is the most important
> thing).
> 
> The guest *does* know what the hypervisor provides. And when we get to
> do this in virtio, we get all the goodness of negotiation as well. The
> existence of the simple ACPI model doesn't hurt that at all.

Maybe it doesn't, at that. E.g. virtio does a copy, acpi doesn't?
I'll ponder compatibility over the weekend.

-- 
MST