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

[David Woodhouse <dwmw2@infradead.org>]

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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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