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

[Michael S. Tsirkin]

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.

Maybe become you insist on using ACPI?
I see a fairly simple way to do it. For example, with virtio:

one vq per CPU, with a single outstanding buffer,
callback copies from the buffer into the userspace
visible memory.

Want me to show you the code?

-- 
MST