Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption status is changed

[Borislav Petkov <bp@alien8.de>]

On Fri, May 14, 2021 at 09:05:23AM +0000, Ashish Kalra wrote:
> Ideally we should fail/stop migration even if a single guest page
> encryption status cannot be notified and that should be the way to
> proceed in this case, the guest kernel should notify the source
> userspace VMM to block/stop migration in this case.

Yap, and what I'm trying to point out here is that if the low level
machinery fails for whatever reason and it cannot recover, we should
propagate that error up the chain so that the user is aware as to why it
failed.

WARN is a good first start but in some configurations those splats don't
even get shown as people don't look at dmesg, etc.

And I think it is very important to propagate those errors properly
because there's a *lot* of moving parts involved in a guest migration
and you have encrypted memory which makes debugging this probably even
harder, etc, etc.

I hope this makes more sense.

> From a practical side, i do see Qemu's migrate_add_blocker() interface
> but that looks to be a static interface and also i don't think it will
> force stop an ongoing migration, is there an existing mechanism
> to inform userspace VMM from kernel about blocking/stopping migration ?

Hmm, so __set_memory_enc_dec() which calls
notify_addr_enc_status_changed() is called by the guest, right, when it
starts migrating.

Can an error value from it be propagated up the callchain so it can be
turned into an error messsage for the guest owner to see?

(I might be way off base here as I have no clue how the whole migration
 machinery is kicked into gear...)

Thx.

-- 
Regards/Gruss,
    Boris.

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