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

[Ashish Kalra <ashish.kalra@amd.com>]

Hello Boris, Paolo,

On Fri, May 14, 2021 at 10:03:18AM +0200, Paolo Bonzini wrote:
> On 14/05/21 09:33, Borislav Petkov wrote:
> > Ok, so explain to me how this looks from the user standpoint: she starts
> > migrating the guest, it fails to lookup an address, there's nothing
> > saying where it failed but the guest crashed.
> > 
> > Do you think this is user-friendly?
> 
> Ok, so explain to me how this looks from the submitter standpoint: he reads
> your review of his patch, he acknowledges your point with "Yes, it makes
> sense to signal it with a WARN or so", and still is treated as shit.
> 
> Do you think this is friendly?
> 
> 

I absolutely agree with both of your point of view. But what's the
alternative ?

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.

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 ?

Thanks,
Ashish