linux-kernel - Re: [PATCH v6 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Message-ID: <f3a51aef-ab0a-b29f-dfa0-5490518d6fcc@redhat.com>
Date:   Wed, 28 Jun 2017 15:48:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v6 3/4] KVM: async_pf: Force a nested vmexit if the
 injected #PF is async_pf



On 28/06/2017 15:38, Radim Krčmář wrote:
>> Radim, Wanpeng,
>>
>> the patch is nice now but I'm still not 100% sure about the live
>> migration part.  Why do we need to pass nested_apf to userspace, but not
>> nested_apf_token?
>
> We do not need it for migration, but unavailable nested_apf_token
> already breaks checkpoint & restore from userspace ... I think the
> cleanest way would be to add a new paravirtual event for nested_apf.
> (Or just keep delaying the apf.)

Indeed.  With Jim's plans to migrate nested virt data, I was wondering
if nested_apf and nested_apf_token would be better placed in that ioctl,
rather than GET/SET_VCPU_EVENTS.

Nested-virt migration is broken anyway until we have Jim's patches, so
there's little point in migrating nested_apf only.  Do you agree?

> Migration does a "async-pf-broadcast" while setting the async-pf MSR on
> destination, which resumes all async-pf waiters.
> Userspace actually has to drop the async-pf event on migration, because
> the destination has invalid nested_apf_token.  (It's a horrible design.)

Yes, this was my question essentially.  I would still migrate
nested_apf_token (as part of nested virt state), and then clear it in
KVM when doing the async-pf broadcast.

Paolo

> nested_apf is not #PF: if we didn't pass nested_apf, then the exception
> would be injected as #PF to L2 after migration.  (Local KVM could
> remember that the #PF is nested_apf and do some ugly hacks.)