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

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Date:   Wed, 28 Jun 2017 22:09:13 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <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

2017-06-28 21:48 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> 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.

Do you mean I should save nested_apf_token by GET_VCPU_EVENTS and
restore it by SET_VCPU_EVENTS? I utilize the place of "u8 pad" in
kvm_vcpu_events to hold nested_apf, however nested_apf_token is
unsigned long.

Regards,
Wanpeng Li