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

[Zenghui Yu]

Hi Marc, Eric,

On 2019/12/20 21:07, Marc Zyngier wrote:
> On 2019-12-20 11:18, 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.
>
> This is a useless HW optimization, where it can avoid reading the
> pending table the very first time you write to this register if
> it is told that it is all zero. A decent ITS implementation
> already has a mechanism to find out about the pending bits by
> looking into the IMPDEF area (the first 1kB) of the pending table.

Yeah, AFAICT this is what Hisilicon has already implemented today.

> PTZ is just yet another way to do the same thing.
>
> This can only happen once in the lifetime of the system (when allocating
> the table), and Linux doesn't really care.

I now get it, thanks for teaching me that!

> As usual, the GIC is setting
> the level of useless complexity pretty high...
>
> > Âvirt/kvm/arm/vgic/vgic-mmio-v3.c | 5 ++++-
> > Â1 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,
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ gpa_t addr, unsigned int len)
> > Â{
> > ÂÂÂÂ struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
> > +ÂÂÂ u64 value = vgic_cpu->pendbaser;
> >
> > -ÂÂÂ return extract_bytes(vgic_cpu->pendbaser, addr & 7, len);
> > +ÂÂÂ value &= ~GICR_PENDBASER_PTZ;
> > +
> > +ÂÂÂ return extract_bytes(value, addr & 7, len);
> > Â}
> >
> > Âstatic void vgic_mmio_write_pendbase(struct kvm_vcpu *vcpu,
>
> Otherwise looks good. I'll queue it with Eric's correction
> to the subject line.

Thanks both and Merry Christmas!

Zenghui