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

[David Woodhouse]

On 16 July 2024 12:54:52 BST, Peter Hilber <peter.hilber at opensynergy.com> wrote:
>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.

This seems like a bug on the guest side. The HW counter needs to be read *within* the (paired, matching) seqcount reads, not before or after.