linux-kernel - Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation selection in the distributor

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Date:   Fri, 20 Mar 2020 11:08:39 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Auger Eric <eric.auger@...hat.com>, Marc Zyngier <maz@...nel.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jason Cooper <jason@...edaemon.net>,
        "Robert Richter" <rrichter@...vell.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "James Morse" <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation
 selection in the distributor

On 2020/3/20 4:38, Auger Eric wrote:
> Hi Marc,
> On 3/19/20 1:10 PM, Marc Zyngier wrote:
>> Hi Zenghui,
>>
>> On 2020-03-18 06:34, Zenghui Yu wrote:
>>> Hi Marc,
>>>
>>> On 2020/3/5 4:33, Marc Zyngier wrote:
>>>> The GICv4.1 architecture gives the hypervisor the option to let
>>>> the guest choose whether it wants the good old SGIs with an
>>>> active state, or the new, HW-based ones that do not have one.
>>>>
>>>> For this, plumb the configuration of SGIs into the GICv3 MMIO
>>>> handling, present the GICD_TYPER2.nASSGIcap to the guest,
>>>> and handle the GICD_CTLR.nASSGIreq setting.
>>>>
>>>> In order to be able to deal with the restore of a guest, also
>>>> apply the GICD_CTLR.nASSGIreq setting at first run so that we
>>>> can move the restored SGIs to the HW if that's what the guest
>>>> had selected in a previous life.
>>>
>>> I'm okay with the restore path.  But it seems that we still fail to
>>> save the pending state of vSGI - software pending_latch of HW-based
>>> vSGIs will not be updated (and always be false) because we directly
>>> inject them through ITS, so vgic_v3_uaccess_read_pending() can't
>>> tell the correct pending state to user-space (the correct one should
>>> be latched in HW).
>>>
>>> It would be good if we can sync the hardware state into pending_latch
>>> at an appropriate time (just before save), but not sure if we can...
>>
>> The problem is to find the "appropriate time". It would require to define
>> a point in the save sequence where we transition the state from HW to
>> SW. I'm not keen on adding more state than we already have.
> 
> may be we could use a dedicated device group/attr as we have for the ITS
> save tables? the user space would choose.

It means that userspace will be aware of some form of GICv4.1 details
(e.g., get/set vSGI state at HW level) that KVM has implemented.
Is it something that userspace required to know? I'm open to this ;-)

> 
> Thanks
> 
> Eric
>>
>> But what we can do is to just ask the HW to give us the right state
>> on userspace access, at all times. How about this:
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> index 48fd9fc229a2..281fe7216c59 100644
>> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>> @@ -305,8 +305,18 @@ static unsigned long
>> vgic_v3_uaccess_read_pending(struct kvm_vcpu *vcpu,
>>        */
>>       for (i = 0; i < len * 8; i++) {
>>           struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
>> +        bool state = irq->pending_latch;
>>
>> -        if (irq->pending_latch)
>> +        if (irq->hw && vgic_irq_is_sgi(irq->intid)) {
>> +            int err;
>> +
>> +            err = irq_get_irqchip_state(irq->host_irq,
>> +                            IRQCHIP_STATE_PENDING,
>> +                            &state);
>> +            WARN_ON(err);
>> +        }
>> +
>> +        if (state)
>>               value |= (1U << i);
>>
>>           vgic_put_irq(vcpu->kvm, irq);

Anyway this looks good to me and will do the right thing on a userspace
save.

>>
>> I can add this to "KVM: arm64: GICv4.1: Add direct injection capability
>> to SGI registers".

Thanks,
Zenghui