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

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Message-ID: <811E8A25-3DBC-452D-B594-F9B7B0B61335@infradead.org>
Date: Fri, 26 Jul 2024 19:28:28 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
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 26 July 2024 17:49:58 BST, Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
>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
>
>



Hm, is the same not true for QEMUVGID and AMZNVGID, which I was using as an example?

QEMU seemed to get to 0002, and AFAICT the VMGENID patches were initially posted using QEMU0003, but what's actually in QEMU now is QEMUVGID. So I presumed that was now the preferred option.