Re: [PATCH v3 14/18] KVM: arm64: nv: Add SW walker for AT S1 emulation

Alexandru Elisei
2024-08-16T11:02:43Z

Hi Marc,

On Fri, Aug 16, 2024 at 11:37:24AM +0100, Marc Zyngier wrote:
> Hi Alex,
>
> On Fri, 16 Aug 2024 10:22:43 +0100,
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> >
> > Hi Marc,
> >
> > On Thu, Aug 15, 2024 at 07:28:41PM +0100, Marc Zyngier wrote:
> > >
> > > Hi Alex,
> > >
> > > On Thu, 15 Aug 2024 17:44:02 +0100,
> > > Alexandru Elisei <alexandru.elisei@arm.com> wrote:
>
> [...]
>
> > > > > +static bool par_check_s1_perm_fault(u64 par)
> > > > > +{
> > > > > + u8 fst = FIELD_GET(SYS_PAR_EL1_FST, par);
> > > > > +
> > > > > + return ((fst & ESR_ELx_FSC_TYPE) == ESR_ELx_FSC_PERM &&
> > > > > + !(par & SYS_PAR_EL1_S));
> > > >
> > > > ESR_ELx_FSC_PERM = 0x0c is a permission fault, level 0, which Arm ARM says can
> > > > only happen when FEAT_LPA2. I think the code should check that the value for
> > > > PAR_EL1.FST is in the interval (ESR_ELx_FSC_PERM_L(0), ESR_ELx_FSC_PERM_L(3)].
> > >
> > > I honestly don't want to second-guess the HW. If it reports something
> > > that is the wrong level, why should we trust the FSC at all?
> >
> > Sorry, I should have been clearer.
> >
> > It's not about the hardware reporting a fault on level 0 of the translation
> > tables, it's about the function returning false if the hardware reports a
> > permission fault on levels 1, 2 or 3 of the translation tables.
> >
> > For example, on a permssion fault on level 3, PAR_EL1. FST = 0b001111 = 0x0F,
> > which means that the condition:
> >
> > (fst & ESR_ELx_FSC_TYPE) == ESR_ELx_FSC_PERM (which is 0x0C) is false and KVM
> > will fall back to the software walker.
> >
> > Does that make sense to you?
>
> I'm afraid I still don't get it.
>
> From the kernel source:
>
> #define ESR_ELx_FSC_TYPE (0x3C)
>
> This is a mask covering all fault types.
>
> #define ESR_ELx_FSC_PERM (0x0C)
>
> This is the value for a permission fault, not encoding a level.
>
> Taking your example:
>
> (fst & ESR_ELx_FSC_TYPE) == (0x0F & 0x3C) == 0x0C == ESR_ELx_FSC_PERM
>
> As I read it, the condition is true, as it catches a permission fault
> on any level between 0 and 3.
>
> You're obviously seeing something I don't, and I'm starting to
> question my own sanity...

No, no, sorry for leading you on a wild goose chase, I read 0x3F for
ESR_ELx_FSC_TYPE, which the value for the variable directly above it, instead of
0x3C :(

My bad, the code is correct!

Thanks,
Alex