Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as RAZ

[Marc Zyngier]

Hi Zenghui,

On 2019-12-23 13:43, Zenghui Yu wrote:
> On 2019/12/20 19:18, Zenghui Yu wrote:
> > Although guest will hardly read and use the PTZ (Pending Table Zero)
> > bit in GICR_PENDBASER, let us emulate the architecture strictly.
> > As per IHI 0069E 9.11.30, PTZ field is WO, and reads as 0.
> > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > ---
> > Noticed when checking all fields of GICR_PENDBASER register.
> > But _not_ sure whether it's worth a fix, as Linux never sets
> > the PTZ bit before enabling LPI (set GICR_CTLR_ENABLE_LPIS).
> > And I wonder under which scenarios can this bit be written as 1.
> > It seems difficult for software to determine whether the pending
> > table contains all zeros when writing this bit.
> > virt/kvm/arm/vgic/vgic-mmio-v3.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> > diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c 
> > b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> > index 7dfd15dbb308..ebc218840fc2 100644
> > --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
> > +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> > @@ -414,8 +414,11 @@ static unsigned long 
> > vgic_mmio_read_pendbase(struct kvm_vcpu *vcpu,
> >   					     gpa_t addr, unsigned int len)
> >   {
> >   	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
> > +	u64 value = vgic_cpu->pendbaser;
> >   -	return extract_bytes(vgic_cpu->pendbaser, addr & 7, len);
> > +	value &= ~GICR_PENDBASER_PTZ;
> > +
> > +	return extract_bytes(value, addr & 7, len);
> >   }
> >   static void vgic_mmio_write_pendbase(struct kvm_vcpu *vcpu,
>
> I noticed there is no userspace access callbacks for GICR_PENDBASER,
> so this patch will make the PTZ field also 'Read As Zero' by 
> userspace.
> Should we consider adding a uaccess_read callback for GICR_PENDBASER
> which just returns the unchanged vgic_cpu->pendbaser to userspace?
> (Though this is really not a big deal. We now always emulate the PTZ
> field to guest as RAZ. And 'vgic_cpu->pendbaser & GICR_PENDBASER_PTZ'
> only indicates whether KVM will optimize the LPI enabling process,
> where Read As Zero indicates never optimize..)

I don't think adding a userspace accessor would help much. All this
bit tells userspace is that the guest has programmed a zero filled
table. On restore, we'd avoid a rescan of the table if there was
no LPI mapped.

And thinking of it, this fixes a bug for non-Linux guests: If you write
PTZ=1, we never clear it. Which means that if userspace saves and 
restores
PENDBASER with PTZ set, we'll never restore the pending bits, which is
pretty bad (see vgic_enable_lpis()).

This patch on its own fixes more than one bug!

Thanks,

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