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

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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.

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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 ;)