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

[Peter Hilber]

On 11.07.24 09:50, David Woodhouse wrote:
> 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

My point is that, looking at the above steps 1. - 5.:

3. read HW counter, smp_rmb, read seqcount
4. store seqcount, smp_wmb, stores, smp_wmb, store seqcount become effective
5. read seqcount, smp_rmb, read HW counter

AFAIU this would still be a theoretical problem suggesting the use of
stronger barriers.

> 
> 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.

Sounds good to me. This statement would then also address the above.