On Thu, 2024-06-20 at 14:01 +0200, Peter Hilber wrote:

On 15.06.24 10:01, David Woodhouse wrote:

quoted

On Mon, 2023-12-18 at 08:38 +0100, Peter Hilber wrote:

quoted

+       ret = viortc_hw_xtstamp_params(&hw_counter, &cs_id);
+       if (ret)
+               return ret;
+
+       ktime_get_snapshot(&history_begin);
+       if (history_begin.cs_id != cs_id)
+               return -EOPNOTSUPP;

I think you have to call ktime_get_snapshot() anyway to get a snapshot
from before your crosststamp? But I still don't much like the fact that
you need to use it to work out which cs_id is being used.

The actual cs_id check is in get_device_system_crosststamp(), where it was
added recently [1]. So this additional check is just verifying that the
history_begin is usable.

quoted

Shouldn't get_device_system_crosststamp() pass that to its get_time_fn
as a hint?

This is unneeded in this case, since get_device_system_crosststamp() does
the check already (but the driver is free to pass it through the
get_time_fn parameter ctx).

The *check* is a different thing.

As things stand, the device has to *choose* a cs_id to use, and takes a
gamble on that check in get_device_system_crosststamp() throwing the
crosststamp away with -ENODEV because the device picked the wrong
cs_id.

That's why I'm saying it would be nicer if the core code *told* the
device what cs_id to use. Rather than just throwing it away if the
device guesses wrong.

(Yes, it would have to be considered a hint, because it could
theoretically have *changed* by the time the result is obtained, just
as with your code above.)

quoted

On x86, you are likely to find that history_begin.cs_id is the KVM
clock, so this will return -EOPNOTSUPP and userspace will have to fall
back to PTP_SYS_OFFSET. I note the KVM PTP clock actually *converts* a
TSC-based crosststamp to kvmclock µs for itself, so that it can report 
*cs_id = CSID_X86_KVM_CLK. Not sure how I feel about that though. I'm
inclined to suggest that it shouldn't, as anyone who wants accurate
timekeeping shouldn't be using the KVM clock anyway.

But we should at least be relatively consistent about it.

ATM, the driver does indeed not have TSC support (for cross-timestamping)
enabled at all, so would always use fallback. If *not* using the KVM clock,
I think TSC can just be enabled by adding architecture-specific code
similar to virtio_rtc_arm.c.

I am not familiar with the KVM clock, but maybe it would be sufficient to
allow CSID_X86_KVM_CLK as well?

Sure, that's what the ptp_kvm clock does. It actually obtains a TSC
reading from the "hardware", and then manually (and unconditionally)
converts that to a kvmclock value so that it can return a clock pairing
based on CSID_X86_KVM_CLK.

Which works until the user configures the clocksource to be the TSC
instead of kvmclock, and then hits that -ENODEV check and has to do the
fallback.

We should just tell the device which cs_id to use.