Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending state to physical side

From: Shenming Lu <lushenming@huawei.com>
To: Auger Eric <eric.auger@redhat.com>, 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>,
	<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
Date: Thu, 17 Dec 2020 12:19:35 +0800	[thread overview]
Message-ID: <5732e2a2-7b78-dcbe-bd7d-77541c7bcf16@huawei.com> (raw)
In-Reply-To: <f8b398df-9945-9ce6-18e6-970637a1bb51@redhat.com>

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
>>>> 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?...
> 
> 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,
Shenming

> 
> 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.
>>
> 
> .
> 