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

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Message-ID: <59ec07e5-c017-8644-b96f-e87fe600c490@huawei.com>
Date:   Tue, 1 Dec 2020 20:15:14 +0800
From:   Shenming Lu <lushenming@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Eric Auger <eric.auger@...hat.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>, Neo Jia <cjia@...dia.com>,
        <wanghaibin.wang@...wei.com>, <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v1 3/4] KVM: arm64: GICv4.1: Restore VLPI's pending
 state to physical side

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.