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

[Zenghui Yu]

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..)


Thanks,
Zenghui