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

[Zenghui Yu <yuzenghui@huawei.com>]

Hi Marc,

On 2020/3/20 17:01, Marc Zyngier wrote:
> Hi Zenghui,
> 
> On 2020-03-20 03:53, Zenghui Yu wrote:
>> Hi Marc,
>>
>> On 2020/3/19 20:10, Marc Zyngier wrote:
>>>> 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.
>>
>> This is exactly what I *was* concerning about.
>>
>>> 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?
>>
>> Let's assume two hosts below. 'has_gicv4_1' is true on host-A while
>> it is false on host-B because of lack of HW support or the kernel
>> parameter "kvm-arm.vgic_v4_enable=0".
>>
>> If we migrate guest through A->B->A, we may end-up lose the initial
>> guest-request "nASSGIreq=1" and don't use direct vSGI delivery for
>> this guest when it's migrated back to host-A.
> 
> My point above is that we shouldn't allow the A->B migration the first
> place, and fail it as quickly as possible. We don't know what the guest
> has observed in terms of GIC capability, and it may not have enabled the
> new flavour of SGIs just yet.

Indeed. I didn't realize it.

> 
>> This can be "fixed" by keep track of what guest had written into
>> nASSGIreq. And we need to evaluate the need for using direct vSGI
>> for a specified guest by 'has_gicv4_1 && nassgireq'.
> 
> It feels odd. It means we have more state than the HW normally has.
> I have an alternative proposal, see below.
> 
>> But if it's expected that "if userspace tries to set nASSGIreq on
>> a non-4.1 host, this bit will not latch", then this shouldn't be
>> a problem at all.
> 
> Well, that is the semantics of the RES0 bit. It applies from both
> guest and userspace.
> 
> And actually, maybe we can handle that pretty cheaply. If userspace
> tries to restore GICD_TYPER2 to a value that isn't what KVM can
> offer, we just return an error. Exactly like we do for GICD_IIDR.
> Hence the following patch:
> 
> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c 
> b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> index 28b639fd1abc..e72dcc454247 100644
> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> @@ -156,6 +156,7 @@ static int vgic_mmio_uaccess_write_v3_misc(struct 
> kvm_vcpu *vcpu,
>       struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
> 
>       switch (addr & 0x0c) {
> +    case GICD_TYPER2:
>       case GICD_IIDR:
>           if (val != vgic_mmio_read_v3_misc(vcpu, addr, len))
>               return -EINVAL;
> 
> Being a RO register, writing something that isn't compatible with the
> possible behaviour of the hypervisor will just return an error.

This is really a nice point to address my concern! I'm happy to see
this in v6 now.

> 
> What do you think?

I agreed with you, with a bit nervous though. Some old guests (which
have no knowledge about GICv4.1 vSGIs and don't care about nASSGIcap
at all) will also fail to migrate from A to B, just because now we
present two different (unused) GICD_TYPER2 registers to them.

Is it a little unfair to them :-) ?


Thanks,
Zenghui