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Subject: Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending
 state to physical side
To:     Marc Zyngier <maz@kernel.org>
CC:     James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Eric Auger <eric.auger@redhat.com>,
        <linux-arm-kernel@lists.infradead.org>,
        <kvmarm@lists.cs.columbia.edu>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
        Christoffer Dall <christoffer.dall@arm.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Cornelia Huck <cohuck@redhat.com>, Neo Jia <cjia@nvidia.com>,
        <wanghaibin.wang@huawei.com>, <yuzenghui@huawei.com>
References: <20201123065410.1915-1-lushenming@huawei.com>
 <20201123065410.1915-4-lushenming@huawei.com>
 <5c724bb83730cdd5dcf7add9a812fa92@kernel.org>
 <b03edcf2-2950-572f-fd31-601d8d766c80@huawei.com>
 <2d2bcae4f871d239a1af50362f5c11a4@kernel.org>
 <49610291-cf57-ff78-d0ac-063af24efbb4@huawei.com>
 <48c10467-30f3-9b5c-bbcb-533a51516dc5@huawei.com>
 <2ad38077300bdcaedd2e3b073cd36743@kernel.org>
 <9b80d460-e149-20c8-e9b3-e695310b4ed1@huawei.com>
 <274dafb2e21f49326a64bb575e668793@kernel.org>
From:   Shenming Lu <lushenming@huawei.com>
Message-ID: <59ec07e5-c017-8644-b96f-e87fe600c490@huawei.com>
Date:   Tue, 1 Dec 2020 20:15:14 +0800
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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
>> KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES...
>> 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?...

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.