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

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Message-ID: <c5fef494365f738bfe02b6afd54d191260a8233b.camel@infradead.org>
Date: Fri, 26 Jul 2024 14:08:38 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
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, 2024-07-26 at 09:04 -0400, Michael S. Tsirkin wrote:
> 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.

For clarity, I think I've ditched the idea of a poor-man's negotiation
through invoking an ACPI method to enable the timekeeping info.

I think we're better off waiting for virtio, to enable that kind of
thing.

The guest gets what the hypervisor is prepared to offer "for free".

That isn't a one-way door; we *can* add an optional ACPI method later
if we really want to. But I'm generally OK with "use virtio if you want
that".

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