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

[David Woodhouse]

On Thu, 2024-07-25 at 17:04 -0400, Michael S. Tsirkin wrote:
> On Thu, Jul 25, 2024 at 10:00:24PM +0100, David Woodhouse wrote:
> > On Thu, 2024-07-25 at 16:50 -0400, Michael S. Tsirkin wrote:
> > > On Thu, Jul 25, 2024 at 08:35:40PM +0100, David Woodhouse wrote:
> > > > On Thu, 2024-07-25 at 12:38 -0400, Michael S. Tsirkin wrote:
> > > > > On Thu, Jul 25, 2024 at 04:18:43PM +0100, David Woodhouse wrote:
> > > > > > The use case isn't necessarily for all users of gettimeofday(), of
> > > > > > course; this is for those applications which *need* precision time.
> > > > > > Like distributed databases which rely on timestamps for coherency, and
> > > > > > users who get fined millions of dollars when LM messes up their clocks
> > > > > > and they put wrong timestamps on financial transactions.
> > > > > 
> > > > > I would however worry that with all this pass through,
> > > > > applications have to be coded to each hypervisor or even
> > > > > version of the hypervisor.
> > > > 
> > > > Yes, that would be a problem. Which is why I feel it's so important to
> > > > harmonise the contents of the shared memory, and I'm implementing it
> > > > both QEMU and $DAYJOB, as well as aligning with virtio-rtc.
> > > 
> > > 
> > > Writing an actual spec for this would be another thing that might help.

Potentially, although working over it with our internal clock team and
with Peter on virtio-rtc has put us in good shape. I'm confident now
that we have something that's viable and extensible enough.

> > > 
> > > > > virtio has been developed with the painful experience that we keep
> > > > > making mistakes, or coming up with new needed features,
> > > > > and that maintaining forward and backward compatibility
> > > > > becomes a whole lot harder than it seems in the beginning.
> > > > 
> > > > Yes. But as you note, this shared memory structure is a userspace ABI
> > > > all of its own, so we get to make a completely *different* kind of
> > > > mistake :)
> > > > 
> > > 
> > > 
> > > So, something I still don't completely understand.
> > > Can't the VDSO thing be written to by kernel?
> > > Let's say on LM, an interrupt triggers and kernel copies
> > > data from a specific device to the VDSO.
> > > 
> > > Is that problematic somehow? I imagine there is a race where
> > > userspace reads vdso after lm but before kernel updated
> > > vdso - is that the concern?

Yes.

> > > Then can't we fix it by interrupting all CPUs right after LM?
> > > 
> > > To me that seems like a cleaner approach - we then compartmentalize
> > > the ABI issue - kernel has its own ABI against userspace,
> > > devices have their own ABI against kernel.
> > > It'd mean we need a way to detect that interrupt was sent,
> > > maybe yet another counter inside that structure.
> > > 
> > > WDYT?
> > > 
> > > By the way the same idea would work for snapshots -
> > > some people wanted to expose that info to userspace, too.

Those people included me. I wanted to interrupt all the vCPUs, even the
ones which were in userspace at the moment of migration, and have the
kernel deal with passing it on to userspace via a different ABI.

It ends up being complex and intricate, and requiring a lot of new
kernel and userspace support. I gave up on it in the end for snapshots,
and didn't go there again for this.

By contrast, a driver which merely exposes a page of MMIO space
identified by an ACPI device (without even the in-kernel PTP support)
could probably be fewer than a hundred lines of code. In an externally-
buildable module that goes back as far as RHEL8 or even further,
allowing users to just build and use it from their application.

> was there supposed to be text here, or did you just like this
> so much you decided to repost my mail ;) 

Hm, weirdness. I've known Evolution get into a state where it sends
completely *empty* messages, but I've never seen it eat only my own
part before. I had definitely typed responses (along the lines of the
above) last time.

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