Re: [PATCH 5/5] KVM: arm/arm64: vgic-v3: Use PREbits to infer the number of ICH_APxRn_EL2 registers

Christoffer Dall
2017-05-15T09:30:14Z

On Tue, May 02, 2017 at 02:30:41PM +0100, Marc Zyngier wrote:
> The GICv3 documentation is extremely confusing, as it talks about
> the number of priorities represented by the ICH_APxRn_EL2 registers,
> while it should really talk about the number of preemption levels.
>
> This leads to a bug where we may access undefined ICH_APxRn_EL2
> registers, since PREbits is allowed to be smaller than PRIbits.

How does this work from the guest's point of view? If I read the spec
correctly, software can derive the number of supported priority levels
(and thereby the minimal value of ICC_BPR0_EL1.BinaryPoint field) from
the number of priority bits implemented, which is exposed via
ICC_CTLR_EL1.PRIbits.

If that minimum value can be higher when running in a VM, does that mean
that an OS that wants to support running in a VM and on real hardware,
must adjust its expectations by writing to the BinaryPoint and read back
the value? Otherwise it seems to me it won't get the preemption it
asked for.

> Thankfully, nobody seem to have taken this path so far...
>
> The fix is to use ICH_VTR_EL2.PREbits instead.

Strictly speaking, I cannot find anything in the spec that says that
this is the way things are connected, although it seems to me that it's
the most obvious thing. Is there a plan to clarify the spec?

>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

I'm going to say that I reviewed this patch because you're potentially a
better authority in this area than the spec.

Reviewed-by: Christoffer Dall <cdall@linaro.org>

> ---
> virt/kvm/arm/hyp/vgic-v3-sr.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
> index bce6037cf01d..32c3295929b0 100644
> --- a/virt/kvm/arm/hyp/vgic-v3-sr.c
> +++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
> @@ -22,7 +22,7 @@
> #include <asm/kvm_hyp.h>
>
> #define vtr_to_max_lr_idx(v) ((v) & 0xf)
> -#define vtr_to_nr_pri_bits(v) (((u32)(v) >> 29) + 1)
> +#define vtr_to_nr_pre_bits(v) (((u32)(v) >> 26) + 1)
>
> static u64 __hyp_text __gic_v3_get_lr(unsigned int lr)
> {
> @@ -135,13 +135,13 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
>
> if (used_lrs) {
> int i;
> - u32 nr_pri_bits;
> + u32 nr_pre_bits;
>
> cpu_if->vgic_elrsr = read_gicreg(ICH_ELSR_EL2);
>
> write_gicreg(0, ICH_HCR_EL2);
> val = read_gicreg(ICH_VTR_EL2);
> - nr_pri_bits = vtr_to_nr_pri_bits(val);
> + nr_pre_bits = vtr_to_nr_pre_bits(val);
>
> for (i = 0; i < used_lrs; i++) {
> if (cpu_if->vgic_elrsr & (1 << i))
> @@ -152,7 +152,7 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
> __gic_v3_set_lr(0, i);
> }
>
> - switch (nr_pri_bits) {
> + switch (nr_pre_bits) {
> case 7:
> cpu_if->vgic_ap0r[3] = read_gicreg(ICH_AP0R3_EL2);
> cpu_if->vgic_ap0r[2] = read_gicreg(ICH_AP0R2_EL2);
> @@ -162,7 +162,7 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
> cpu_if->vgic_ap0r[0] = read_gicreg(ICH_AP0R0_EL2);
> }
>
> - switch (nr_pri_bits) {
> + switch (nr_pre_bits) {
> case 7:
> cpu_if->vgic_ap1r[3] = read_gicreg(ICH_AP1R3_EL2);
> cpu_if->vgic_ap1r[2] = read_gicreg(ICH_AP1R2_EL2);
> @@ -198,7 +198,7 @@ void __hyp_text __vgic_v3_restore_state(struct kvm_vcpu *vcpu)
> struct vgic_v3_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v3;
> u64 used_lrs = vcpu->arch.vgic_cpu.used_lrs;
> u64 val;
> - u32 nr_pri_bits;
> + u32 nr_pre_bits;
> int i;
>
> /*
> @@ -217,12 +217,12 @@ void __hyp_text __vgic_v3_restore_state(struct kvm_vcpu *vcpu)
> }
>
> val = read_gicreg(ICH_VTR_EL2);
> - nr_pri_bits = vtr_to_nr_pri_bits(val);
> + nr_pre_bits = vtr_to_nr_pre_bits(val);
>
> if (used_lrs) {
> write_gicreg(cpu_if->vgic_hcr, ICH_HCR_EL2);
>
> - switch (nr_pri_bits) {
> + switch (nr_pre_bits) {
> case 7:
> write_gicreg(cpu_if->vgic_ap0r[3], ICH_AP0R3_EL2);
> write_gicreg(cpu_if->vgic_ap0r[2], ICH_AP0R2_EL2);
> @@ -232,7 +232,7 @@ void __hyp_text __vgic_v3_restore_state(struct kvm_vcpu *vcpu)
> write_gicreg(cpu_if->vgic_ap0r[0], ICH_AP0R0_EL2);
> }
>
> - switch (nr_pri_bits) {
> + switch (nr_pre_bits) {
> case 7:
> write_gicreg(cpu_if->vgic_ap1r[3], ICH_AP1R3_EL2);
> write_gicreg(cpu_if->vgic_ap1r[2], ICH_AP1R2_EL2);
> --
> 2.11.0
>