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

[Marc Zyngier]

Hi Zenghui,

[...]

> > 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,
> >  ÂÂÂÂ struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
> >
> >  ÂÂÂÂ switch (addr & 0x0c) {
> > +ÂÂÂ case GICD_TYPER2:
> >  ÂÂÂÂ case GICD_IIDR:
> >  ÂÂÂÂÂÂÂÂ if (val != vgic_mmio_read_v3_misc(vcpu, addr, len))
> >  ÂÂÂÂÂÂÂÂÂÂÂÂ 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 :-) ?

I never pretended to be fair! ;-)

I'm happy to prevent migrating from a v4.1 system (A) to a v4.0
system (B). As soon as the guest has run, it isn't safe to do so
(it may have read TYPER2, and now knows about vSGIs). We *could*
track this and find ways to migrate this state as well, but it
feels fragile.

Migrating from B to A is more appealing. It should be possible to
do so without much difficulty (just check that the nASSGIcap bit
is either 0 or equal to KVM's view of the capability).

But overall I seriously doubt we can easily migrate guests across
very different HW. We've been talking about this for years, and
we still don't have a good solution for it given the diversity
of the ecosystem... :-/

Thanks,

        M.
--
Jazz is not dead. It just smells funny...