RE: [PATCH] doc/arm: take care restore order of GICR_* in ITS restore

[Jianyong Wu]

Hi Marc,

Get it! Thanks for your explanation.

Thanks
Jianyong

> -----Original Message-----
> From: Marc Zyngier <maz@xxxxxxxxxx>
> Sent: Thursday, July 22, 2021 4:11 PM
> To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
> Cc: James Morse <James.Morse@xxxxxxx>; Andre Przywara
> <Andre.Przywara@xxxxxxx>; lushenming@xxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx; linux-
> doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Justin He
> <Justin.He@xxxxxxx>
> Subject: Re: [PATCH] doc/arm: take care restore order of GICR_* in ITS
> restore
>
> On Thu, 22 Jul 2021 03:49:52 +0100,
> Jianyong Wu <Jianyong.Wu@xxxxxxx> wrote:
> >
> > Hello Marc,
> >
> > > -----Original Message-----
> > > From: Marc Zyngier <maz@xxxxxxxxxx>
> > > Sent: Wednesday, July 21, 2021 5:54 PM
> > > To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
> > > Cc: James Morse <James.Morse@xxxxxxx>; Andre Przywara
> > > <Andre.Przywara@xxxxxxx>; lushenming@xxxxxxxxxx;
> > > kvm@xxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx; linux-
> > > doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Justin He
> > > <Justin.He@xxxxxxx>
> > > Subject: Re: [PATCH] doc/arm: take care restore order of GICR_* in
> > > ITS restore
> > >
> > > On Wed, 21 Jul 2021 10:20:19 +0100,
> > > Jianyong Wu <jianyong.wu@xxxxxxx> wrote:
> > > >
> > > > When restore GIC/ITS, GICR_CTLR must be restored after
> > > > GICR_PROPBASER and GICR_PENDBASER. That is important, as both of
> > > > GICR_PROPBASER and GICR_PENDBASER will fail to be loaded when lpi
> > > > has enabled yet in GICR_CTLR. Keep the restore order above will avoid
> that issue.
> > > > Shout it out at the doc is very helpful that may avoid lots of debug work.
> > >
> > > But that's something that is already mandated by the architecture, isn't it?
> > > See "5.1 LPIs" in the architecture spec:
> > >
> > > <quote>
> > >
> > > If GICR_PROPBASER is updated when GICR_CTLR.EnableLPIs == 1, the
> > > effects are UNPREDICTABLE.
> > >
> > > [...]
> > >
> > > If GICR_PENDBASER is updated when GICR_CTLR.EnableLPIs == 1, the
> > > effects are UNPREDICTABLE.
> > >
> >
> > I think this "UNPREDICTABLE" related with the "physical machine". Am I
> > right?
>
> No, you are unfortunately wrong. The architecture applies to *any*
> implementation, and makes no distinction between a HW implementation of
> a SW version. This is why we call it an architecture, and not an
> implementation.
>
> > In virtualization environment, kernel gives the definite answer that
> > we should not enable GICR_CTLR.EnableLPIs before restoring
> > GICR_PROPBASER(GICR_PENDBASER either) when restore GIC ITS in VMM,
> see
> > [1]. Thus, should we consider the virtualization environment as a
> > special case?
>
> Absolutely not.  If you start special casing things, then we end-up having
> stupidly designed SW that tries to do stupid things based on the supposed
> properties of an implementation.
>
> We follow the architecture, all the architecture, nothing but the architecture.
> The architecture is the only barrier between insanity and pure madness! ;-)
>
> >
> > [1] linux/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> > static void vgic_mmio_write_propbase(struct kvm_vcpu *vcpu,
> >                                      gpa_t addr, unsigned int len,
> >                                      unsigned long val) {
> >         struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
> >         struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
> >         u64 old_propbaser, propbaser;
> >
> >         /* Storing a value with LPIs already enabled is undefined */
> >         if (vgic_cpu->lpis_enabled)
> >            return;
> > ...
> > }
>
> Do you see how the kernel does exactly what the architecture says we can do?
> Ignoring the write is a perfectly valid implementation of UNPREDICTABLE.
>
> So what we do is completely in line with the architecture. As such, no need to
> document it any further, everything is already where it should be. If
> someone tries to write code dealing with the GIC without understanding the
> architecture, no amount of additional documentation will help.
>
> Thanks,
>
>       M.
>
> --
> Without deviation from the norm, progress is not possible.
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