Re: [PATCH 0/5] KVM: x86: Fix breakage in KVM_SET_XSAVE's ABI

[Tyler Stachecki <stachecki.tyler@gmail.com>]

On Wed, Oct 04, 2023 at 04:11:52AM -0300, Leonardo Bras wrote:
> So this patch is supposed to fix migration of VM from a host with
> pre-ad856280ddea (OLD) kernel to a host with ad856280ddea + your set(NEW).
> Right?
> 
> Let's get the scenario here, where all machines are the same:
> 1 - VM created on OLD kernel with a host-supported xfeature F, which is not
>     guest supported.
> 2 - VM is migrated to a NEW kernel/host, and KVM_SET_XSAVE xfeature F.
> 3 - VM will be migrated to another host, qemu requests KVM_GET_XSAVE, which
>     returns only guest-supported xfeatures, and this is passed to next host
> 4 - VM will be started on 3rd host with guest-supported xfeatures, meaning
>     xfeature F is filtered-out, which is not good, because the VM will have
>     less features compared to boot.

This is what I was (trying) to convey earlier...

See Sean's response here:
https://lore.kernel.org/all/ZRMHY83W%2FVPjYyhy@google.com/

I'll copy the pertinent part of his very detailed response inline:
> KVM *must* "trim" features when servicing KVM_GET_SAVE{2}, because that's been
> KVM's ABI for a very long time, and userspace absolutely relies on that
> functionality to ensure that a VM can be migrated within a pool of heterogenous
> systems so long as the features that are *exposed* to the guest are supported
> on all platforms.

My 2 cents: as an outsider with less familiarity of the KVM code, it is hard
to understand the contract here with the guest/userspace. It seems there is a
fundamental question of whether or not "superfluous" features, those being
host-supported features which extend that which the guest is actually capable
of, can be removed between the time that the guest boots and when it
terminates, through however many live-migrations that may be.

Ultimately, this problem is not really fixable if said features cannot be
removed.

Is there an RFC or document which captures expectations of this form?