netdev - Re: [EXTERNAL] [PATCH v6 net-next 0/4] PHC support in ENA driver

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Message-ID: <61bfea4254bfa9a3a64e71ec39c3a1084195856c.camel@infradead.org>
Date: Fri, 14 Feb 2025 09:57:56 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Maciek Machnikowski <maciek@...hnikowski.net>, Jakub Kicinski
	 <kuba@...nel.org>, Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: David Arinzon <darinzon@...zon.com>, David Miller <davem@...emloft.net>,
  netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Richard Cochran
 <richardcochran@...il.com>, "Woodhouse, David" <dwmw@...zon.com>, Rahul
 Rameshbabu <rrameshbabu@...dia.com>, Gal Pressman <gal@...dia.com>
Subject: Re: [EXTERNAL] [PATCH v6 net-next 0/4] PHC support in ENA driver

On Tue, 2025-02-11 at 08:58 +0100, Maciek Machnikowski wrote:
> On 2/11/2025 1:46 AM, Jakub Kicinski wrote:
> > On Sun, 9 Feb 2025 12:33:24 +0000 Vadim Fedorenko wrote:
> > > Yes, we have seen this patchset, and we were thinking of how to
> > > generalize error_bound property, which was removed from the latest
> > > version unfortunately. But it's a good point to look at it once
> > > again in terms of our prototype, thanks!
> > 
> > I was wondering whether they have a user space "time extrapolation
> > component" which we should try to be compatible with. Perhaps they
> > just expect that the user will sync system time.
> 
> error_bound has a different purpose - it tries to get the "baseline"
> clock accuracy from the HW. The number returned here is needed to
> calculate the uncertainty, not to extrapolate.
> 
> And yes - AFIK AWS suggests system time sync for EC2 instances [1]
> 
> [1]
> https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configure-ec2-ntp.html

Right, error bound gives you a min/max for what the time can possibly
be, and that allows you to know whether certain transactions/timestamps
could *possibly* overlap, or if there is a known ordering between them.

There are libraries in userspace which handle this, to be used by
things like distributed databases. https://github.com/aws/clock-bound

The error bound is also exposed by the vmclock device which is now
supported in Linux (https://git.kernel.org/torvalds/c/205032724226) and
QEMU (https://gitlab.com/qemu-project/qemu/-/commit/3634039b93cc5),
although QEMU doesn't actually expose time through it yet, only the
fact that time has been *disrupted* e.g. through live migration).

(The PHC discussed here allows each guest on the system to do the same
work of carefully calibrating the *same* underlying hardware
oscillator; with vmclock the host does it once and then just tells the
guests the result through a shared memory structure. With the added
bonus of still being accurate immediately after live migration to a new
host.)

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