Re: [Qemu-devel] [PATCH RFC 3/8] target-arm: Add support for S2 page-table protection bits

["Edgar E. Iglesias" <edgar.iglesias@xilinx.com>]

On Wed, Sep 23, 2015 at 09:55:05AM -0700, Peter Maydell wrote:
> On 19 September 2015 at 07:15, Edgar E. Iglesias
> <edgar.iglesias@gmail.com> wrote:
> > From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
> >
> > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> > ---
> >  target-arm/helper.c | 48 +++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 45 insertions(+), 3 deletions(-)
> >
> > diff --git a/target-arm/helper.c b/target-arm/helper.c
> > index 33be8c2..6f0ed51 100644
> > --- a/target-arm/helper.c
> > +++ b/target-arm/helper.c
> > @@ -6008,6 +6008,38 @@ simple_ap_to_rw_prot(CPUARMState *env, ARMMMUIdx mmu_idx, int ap)
> >      return simple_ap_to_rw_prot_is_user(ap, regime_is_user(env, mmu_idx));
> >  }
> >
> > +/* Translate S2 section/page access permissions to protection flags
> > + *
> > + * @env:     CPUARMState
> > + * @ap:      The 2-bit simple AP (AP[2:1])
> 
> I think this should read "The 2-bit stage2 access permissions (S2AP)".
> The interpretation of the field differs from the simple-AP bits.
> You could helpfully call the argument "s2ap" as well.
> 
> > + * @xn:      XN (execute-never) bit
> > + */
> > +static int get_S2prot(CPUARMState *env, int ap, int xn)
> > +{
> > +    int prot_rw;
> > +
> > +    switch (ap) {
> > +    default:
> > +    case 0:
> > +        prot_rw = 0;
> > +        break;
> > +    case 1:
> > +        prot_rw = PAGE_READ | PAGE_EXEC;
> > +        break;
> > +    case 2:
> > +        prot_rw = PAGE_WRITE;
> > +        break;
> > +    case 3:
> > +        prot_rw = PAGE_READ | PAGE_EXEC | PAGE_WRITE;
> > +        break;
> > +    }
> > +
> > +    if (xn) {
> > +        prot_rw &= ~PAGE_EXEC;
> > +    }
> 
> This isn't right -- the XN bit controls executability and the
> S2AP bits don't affect it at all. I think you want:
> 
>     int prot_rw = 0;
>     if (s2ap & 1) {
>         prot_rw |= PAGE_READ;
>     }
>     if (s2ap & 2) {
>         prot_rw |= PAGE_WRITE;
>     }
>     if (!xn) {
>         prot_rw |= PAGE_EXEC;
>     }


Thanks, this was the stuff I was worried about when we talked about it
last time. I've got the following now which seems to be the same as
you suggest:

static int get_S2prot(CPUARMState *env, int s2ap, int xn)
{
    int prot;

    prot = s2ap & 1 ? PAGE_READ : 0;
    prot |= s2ap & 2 ? PAGE_WRITE : 0;
    if (!xn) {
        prot |= PAGE_EXEC;
    }
    return prot;
}


I've also fixed up your other comments on this.

Thanks,
Edgar


> 
> 
> > +    return prot_rw;
> > +}
> > +
> >  /* Translate section/page access permissions to protection flags
> >   *
> >   * @env:     CPUARMState
> > @@ -6617,6 +6649,11 @@ static bool get_phys_addr_lpae(CPUARMState *env, target_ulong address,
> >          /* Extract attributes from the descriptor and merge with table attrs */
> >          attrs = extract64(descriptor, 2, 10)
> >              | (extract64(descriptor, 52, 12) << 10);
> > +
> > +        if (mmu_idx == ARMMMUIdx_S2NS) {
> > +            /* The following extractions do not apply to S2.  */
> 
> I think it would be clearer to say
>   /* Stage 2 table descriptors do not include any attribute fields. */
> 
> > +            break;
> > +        }
> 
> and then here put
>       /* Merge in attributes from table descriptors */
> 
> and delete the now-redundant "and merge with table attrs" part from the
> comment at the start of this hunk.
> 
> >          attrs |= extract32(tableattrs, 0, 2) << 11; /* XN, PXN */
> >          attrs |= extract32(tableattrs, 3, 1) << 5; /* APTable[1] => AP[2] */
> >          /* The sense of AP[1] vs APTable[0] is reversed, as APTable[0] == 1
> > @@ -6638,11 +6675,16 @@ static bool get_phys_addr_lpae(CPUARMState *env, target_ulong address,
> >      }
> >
> >      ap = extract32(attrs, 4, 2);
> > -    ns = extract32(attrs, 3, 1);
> >      xn = extract32(attrs, 12, 1);
> > -    pxn = extract32(attrs, 11, 1);
> >
> > -    *prot = get_S1prot(env, mmu_idx, va_size == 64, ap, ns, xn, pxn);
> > +    if (mmu_idx == ARMMMUIdx_S2NS) {
> > +        ns = true;
> > +        *prot = get_S2prot(env, ap, xn);
> > +    } else {
> > +        ns = extract32(attrs, 3, 1);
> > +        pxn = extract32(attrs, 11, 1);
> > +        *prot = get_S1prot(env, mmu_idx, va_size == 64, ap, ns, xn, pxn);
> > +    }
> >
> >      fault_type = permission_fault;
> >      if (!(*prot & (1 << access_type))) {
> > --
> > 1.9.1
> >
> 
> thanks
> -- PMM