linux-kernel - Re: [RFC PATCH v4] ptp: Add vDSO-style vmclock support

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Message-ID: <1ca48fb47723ed16f860611ac230ded7a1ca07f1.camel@infradead.org>
Date: Thu, 11 Jul 2024 08:50:47 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: 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>
Cc: "Christopher S . Hall" <christopher.s.hall@...el.com>, Jason Wang
 <jasowang@...hat.com>, John Stultz <jstultz@...gle.com>, "Michael S .
 Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, Richard Cochran
 <richardcochran@...il.com>, 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: [RFC PATCH v4] ptp: Add vDSO-style vmclock support

On Thu, 2024-07-11 at 09:25 +0200, Peter Hilber wrote:
> 
> IMHO this phrasing is better, since it directly refers to the state of the
> structure.

Thanks. I'll update it.

> AFAIU if there would be abnormal delays in store buffers, causing some
> driver to still see the old clock for some time, the monotonicity could be
> violated:
> 
> 1. device writes new, much slower clock to store buffer
> 2. some time passes
> 3. driver reads old, much faster clock
> 4. device writes store buffer to cache
> 5. driver reads new, much slower clock
> 
> But I hope such delays do not occur.

For the case of the hypervisor←→guest interface this should be handled
by the use of memory barriers and the seqcount lock.

The guest driver reads the seqcount, performs a read memory barrier,
then reads the contents of the structure. Then performs *another* read
memory barrier, and checks the seqcount hasn't changed:
https://git.infradead.org/?p=users/dwmw2/linux.git;a=blob;f=drivers/ptp/ptp_vmclock.c;hb=vmclock#l351

The converse happens with write barriers on the hypervisor side:
https://git.infradead.org/?p=users/dwmw2/qemu.git;a=blob;f=hw/acpi/vmclock.c;hb=vmclock#l68

Do we need to think harder about the ordering across a real PCI bus? It
isn't entirely unreasonable for this to be implemented in hardware if
we eventually add a counter_id value for a bus-visible counter like the
Intel Always Running Timer (ART). I'm also OK with saying that device
implementations may only provide the shared memory structure if they
can ensure memory ordering.

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