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Date:   Thu, 17 Dec 2020 12:19:35 +0800
From:   Shenming Lu <>
To:     Auger Eric <>, Marc Zyngier <>
CC:     James Morse <>,
        Julien Thierry <>,
        Suzuki K Poulose <>,
        <>, <>,
        Christoffer Dall <>,
        Alex Williamson <>,
        Kirti Wankhede <>,
        Cornelia Huck <>, "Neo Jia" <>,
        <>, <>
Subject: Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending
 state to physical side

On 2020/12/16 18:35, Auger Eric wrote:
> Hi Shenming,
> On 12/1/20 1:15 PM, Shenming Lu wrote:
>> On 2020/12/1 19:50, Marc Zyngier wrote:
>>> On 2020-12-01 11:40, Shenming Lu wrote:
>>>> On 2020/12/1 18:55, Marc Zyngier wrote:
>>>>> On 2020-11-30 07:23, Shenming Lu wrote:
>>>>> Hi Shenming,
>>>>>> We are pondering over this problem these days, but still don't get a
>>>>>> good solution...
>>>>>> Could you give us some advice on this?
>>>>>> Or could we move the restoring of the pending states (include the sync
>>>>>> from guest RAM and the transfer to HW) to the GIC VM state change handler,
>>>>>> which is completely corresponding to save_pending_tables (more symmetric?)
>>>>>> and don't expose GICv4...
>>>>> What is "the GIC VM state change handler"? Is that a QEMU thing?
>>>> Yeah, it is a a QEMU thing...
>>>>> We don't really have that concept in KVM, so I'd appreciate if you could
>>>>> be a bit more explicit on this.
>>>> My thought is to add a new interface (to QEMU) for the restoring of
>>>> the pending states, which is completely corresponding to
>>>> And it is called from the GIC VM state change handler in QEMU, which
>>>> is happening after the restoring (call kvm_vgic_v4_set_forwarding())
>>>> but before the starting (running) of the VFIO device.
>>> Right, that makes sense. I still wonder how much the GIC save/restore
>>> stuff differs from other architectures that implement similar features,
>>> such as x86 with VT-D.
>> I am not familiar with it...
>>> It is obviously too late to change the userspace interface, but I wonder
>>> whether we missed something at the time.
>> The interface seems to be really asymmetrical?...
> in qemu d5aa0c229a ("hw/intc/arm_gicv3_kvm: Implement pending table
> save") commit message, it is traced:
> "There is no explicit restore as the tables are implicitly sync'ed
> on ITS table restore and on LPI enable at redistributor level."
> At that time there was no real justification behind adding the RESTORE
> fellow attr.
> Maybe a stupid question but isn't it possible to unset the forwarding
> when saving and rely on VFIO to automatically restore it when resuming
> on destination?

It seems that the unset_forwarding would not be called when saving, it would
be called after migration completion...
As for the resuming/set_forwarding, I still wonder: is it really improper to
transfer the pending states from vgic to VPT in set_forwarding (not only in
migration)?...  -_-


> Thanks
> Eric
>> Or is there a possibility that we could know which irq is hw before the VFIO
>> device calls kvm_vgic_v4_set_forwarding()?
>> Thanks,
>> Shenming
>>> Thanks,
>>>         M.
> .

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