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

[Auger Eric]

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.

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);
> 
> I can add this to "KVM: arm64: GICv4.1: Add direct injection capability
> to SGI registers".
> 
>>
>>>
>>> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
>>> ---
>>>   virt/kvm/arm/vgic/vgic-mmio-v3.c | 48 ++++++++++++++++++++++++++++++--
>>>   virt/kvm/arm/vgic/vgic-v3.c      |  2 ++
>>>   2 files changed, 48 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>> b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>> index de89da76a379..442f3b8c2559 100644
>>> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>> @@ -3,6 +3,7 @@
>>>    * VGICv3 MMIO handling functions
>>>    */
>>>   +#include <linux/bitfield.h>
>>>   #include <linux/irqchip/arm-gic-v3.h>
>>>   #include <linux/kvm.h>
>>>   #include <linux/kvm_host.h>
>>> @@ -70,6 +71,8 @@ static unsigned long vgic_mmio_read_v3_misc(struct
>>> kvm_vcpu *vcpu,
>>>           if (vgic->enabled)
>>>               value |= GICD_CTLR_ENABLE_SS_G1;
>>>           value |= GICD_CTLR_ARE_NS | GICD_CTLR_DS;
>>> +        if (kvm_vgic_global_state.has_gicv4_1 && vgic->nassgireq)
>>
>> Looking at how we handle the GICD_CTLR.nASSGIreq setting, I think
>> "nassgireq==true" already indicates "has_gicv4_1==true".  So this
>> can be simplified.
> 
> Indeed. I've now dropped the has_gicv4.1 check.
> 
>> But I wonder that should we use nassgireq to *only* keep track what
>> the guest had written into the GICD_CTLR.nASSGIreq.  If not, we may
>> lose the guest-request bit after migration among hosts with different
>> has_gicv4_1 settings.
> 
> I'm unsure of what you're suggesting here. If userspace tries to set
> GICD_CTLR.nASSGIreq on a non-4.1 host, this bit will not latch.
> Userspace can check that at restore time. Or we could fail the
> userspace write, which is a bit odd (the bit is otherwise RES0).
> 
> Could you clarify your proposal?
> 
>> The remaining patches all look good to me :-). I will wait for you to
>> confirm these two concerns.
> 
> Thanks,
> 
>         M.