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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2024 at 10:29:18PM +0100, David Woodhouse wrote:
> > > > 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.

ok I believe you, I am just curious how come you need userspace
support - what I imagine would live completely in kernel ...


> 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.

mutt sucks less ;)