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

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Message-ID: <20240726174958.00007d10@Huawei.com>
Date: Fri, 26 Jul 2024 17:49:58 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.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, 25 Jul 2024 14:50:50 +0100
David Woodhouse <dwmw2@...radead.org> wrote:

> On Thu, 2024-07-25 at 08:33 -0400, Michael S. Tsirkin wrote:
> > On Thu, Jul 25, 2024 at 01:31:19PM +0100, David Woodhouse wrote:  
> > > On Thu, 2024-07-25 at 08:29 -0400, Michael S. Tsirkin wrote:  
> > > > On Thu, Jul 25, 2024 at 01:27:49PM +0100, David Woodhouse wrote:  
> > > > > On Thu, 2024-07-25 at 08:17 -0400, Michael S. Tsirkin wrote:  
> > > > > > On Thu, Jul 25, 2024 at 10:56:05AM +0100, David Woodhouse wrote:  
> > > > > > > > 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).
> > > > > > >   
> > > > > > 
> > > > > > Hmm, should we support virtio over ACPI? Just asking.  
> > > > > 
> > > > > Given that we support virtio DT bindings, and the ACPI "PRP0001" device
> > > > > exists with a DSM method which literally returns DT properties,
> > > > > including such properties as "compatible=virtio,mmio" ... do we
> > > > > already?
> > > > > 
> > > > >   
> > > > 
> > > > In a sense, but you are saying that is too complex?
> > > > Can you elaborate?  
> > > 
> > > No, I think it's fine. I encourage the use of the PRP0001 device to
> > > expose DT devices through ACPI. I was just reminding you of its
> > > existence.  
> > 
> > Confused. You said "I know, you can do virtio without PCI but that's
> > complex in other ways" as the explanation why you are doing a custom
> > protocol.  
> 
> Ah, apologies, I wasn't thinking that far back in the conversation.
> 
> If we wanted to support virtio over ACPI, I think PRP0001 can be made
> to work and isn't too complex (even though it probably doesn't yet work
> out of the box).
> 
> But for the VMCLOCK thing, yes, the simple ACPI device is a lot simpler
> than virtio-rtc and much more attractive.
> 
> Even if the virtio-rtc specification were official today, and I was
> able to expose it via PCI, I probably wouldn't do it that way. There's
> just far more in virtio-rtc than we need; the simple shared memory
> region is perfectly sufficient for most needs, and especially ours.
> 
> I have reworked
> https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/vmclock
> to take your other feedback into account.
> 
> It's now more flexible about the size handling, and explicitly checking
> that specific fields are present before using them. 
> 
> I think I'm going to add a method on the ACPI device to enable the
> precise clock information. I haven't done that in the driver yet; it
> still just consumes the precise clock information if it happens to be
> present already. The enable method can be added in a compatible fashion
> (the failure mode is that guests which don't invoke this method when
> the hypervisor needs them to will see only the disruption signal and
> not precise time).
> 
> For the HID I'm going to use AMZNVCLK. I had used QEMUVCLK in the QEMU
> patches, but I'll change that to use AMZNVCLK too when I repost the
> QEMU patch.

That doesn't fit with ACPI _HID definitions.
Second set 4 characters need to be hex digits as this is an
ACPI style ID (which I assume this is given AMZN is a valid
vendor ID.  6.1.5 in ACPI v6.5

Maybe I'm missing something...

J