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.

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