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Date:   Tue, 01 Dec 2020 11:50:00 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     Shenming Lu <lushenming@huawei.com>
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
Subject: Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending
 state to physical side
In-Reply-To: <9b80d460-e149-20c8-e9b3-e695310b4ed1@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>
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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.

It is obviously too late to change the userspace interface, but I wonder
whether we missed something at the time.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...