Re: [Qemu-devel] [PATCH RFC 2/8] target-arm: Add computation of starting level for S2 PTW

[Peter Maydell]

On 19 September 2015 at 07:15, Edgar E. Iglesias
<address@hidden> wrote:
> From: "Edgar E. Iglesias" <address@hidden>
>
> The starting level for S2 pagetable walks is computed
> differently from the S1 starting level. Implement the S2
> variant.
>
> Signed-off-by: Edgar E. Iglesias <address@hidden>
> ---
>  target-arm/helper.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/target-arm/helper.c b/target-arm/helper.c
> index b709582..33be8c2 100644
> --- a/target-arm/helper.c
> +++ b/target-arm/helper.c
> @@ -6542,18 +6542,26 @@ static bool get_phys_addr_lpae(CPUARMState *env, 
> target_ulong address,
>          goto do_fault;
>      }
>
> -    /* The starting level depends on the virtual address size (which can be
> -     * up to 48 bits) and the translation granule size. It indicates the 
> number
> -     * of strides (granule_sz bits at a time) needed to consume the bits
> -     * of the input address. In the pseudocode this is:
> -     *  level = 4 - RoundUp((inputsize - grainsize) / stride)
> -     * where their 'inputsize' is our 'va_size - tsz', 'grainsize' is
> -     * our 'granule_sz + 3' and 'stride' is our 'granule_sz'.
> -     * Applying the usual "rounded up m/n is (m+n-1)/n" and simplifying:
> -     *     = 4 - (va_size - tsz - granule_sz - 3 + granule_sz - 1) / 
> granule_sz
> -     *     = 4 - (va_size - tsz - 4) / granule_sz;
> -     */
> -    level = 4 - (va_size - tsz - 4) / granule_sz;
> +    if (mmu_idx == ARMMMUIdx_S2NS) {
> +        unsigned int startlevel = extract32(tcr->raw_tcr, 6, 2);
> +        level = 3 - startlevel;
> +        if (granule_sz == 9) {
> +            level = 2 - startlevel;
> +        }

I think this is right code-wise but we could make it read a little more
nicely: if you make the condition be "if (mmu_idx != ARMMUIdx_S2NS)" then
the common case comes first and its long comment works as a description
of what we're doing here. Then the else clause can just say
 /* For stage 2 translations the starting level is specified by the
  * VCTR_EL2.SL0 field (whose interpretation depends on the page size)
  */

I was pondering whether writing it as
     if (granule_sz == 9) {
         /* 4K pages */
         level = 2 - startlevel;
     } else {
         /* 16K or 64K pages */
         level = 3 - startlevel;
     }

would be slightly better, but it's marginal. Do add a "4K pages"
comment in somewhere, though.

> +    } else {
> +        /* The starting level depends on the virtual address size (which can
> +         * be up to 48 bits) and the translation granule size. It indicates
> +         * the number of strides (granule_sz bits at a time) needed to
> +         * consume the bits of the input address. In the pseudocode this is:
> +         *  level = 4 - RoundUp((inputsize - grainsize) / stride)
> +         * where their 'inputsize' is our 'va_size - tsz', 'grainsize' is
> +         * our 'granule_sz + 3' and 'stride' is our 'granule_sz'.
> +         * Applying the usual "rounded up m/n is (m+n-1)/n" and simplifying:
> +         * = 4 - (va_size - tsz - granule_sz - 3 + granule_sz - 1) / 
> granule_sz
> +         * = 4 - (va_size - tsz - 4) / granule_sz;
> +         */
> +        level = 4 - (va_size - tsz - 4) / granule_sz;
> +    }
>
>      /* Clear the vaddr bits which aren't part of the within-region address,
>       * so that we don't have to special case things when calculating the
> --
> 1.9.1
>

thanks
-- PMM