Re: [Qemu-devel] [PATCH] PPC: Fix large page support in TCG

[David Gibson]

On Sat, Mar 03, 2012 at 10:39:34AM -0600, Nathan Whitehorn wrote:
> Fix large page support in TCG. The old code would overwrite the
> large page table entry with the fake 4 KB
> one generated here whenever the ref/change bits were updated,
> causing it to point to the wrong area of memory. Instead of creating
> a fake PTE, just update the real address at the end.
> 
> Signed-off-by: Nathan Whitehorn <address@hidden>

Hrm.  This looks like a cleaner way of handling things, but I don't
really follow what exactly was going wrong in the old way.  Can you
spell out in more detail where the modified pte1 value caused
problems?

> ---
>  target-ppc/helper.c |   11 +++++------
>  1 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/target-ppc/helper.c b/target-ppc/helper.c
> index 928fbcf..0f5ad2e 100644
> --- a/target-ppc/helper.c
> +++ b/target-ppc/helper.c
> @@ -597,12 +597,6 @@ static inline int _find_pte(CPUState *env,
> mmu_ctx_t *ctx,
> int is_64b, int h,
>                  pte1 = ldq_phys(env->htab_base + pteg_off + (i * 16) + 8);
>              }
> 
> -            /* We have a TLB that saves 4K pages, so let's
> -             * split a huge page to 4k chunks */
> -            if (target_page_bits != TARGET_PAGE_BITS)
> -                pte1 |= (ctx->eaddr & (( 1 << target_page_bits ) - 1))
> - & TARGET_PAGE_MASK;
> -
>              r = pte64_check(ctx, pte0, pte1, h, rw, type);
>              LOG_MMU("Load pte from " TARGET_FMT_lx " => "
> TARGET_FMT_lx " "
>                      TARGET_FMT_lx " %d %d %d " TARGET_FMT_lx "\n",
> @@ -678,6 +672,11 @@ static inline int _find_pte(CPUState *env,
> mmu_ctx_t *ctx,
> int is_64b, int h,
>          }
>      }
> 
> +    /* We have a TLB that saves 4K pages, so let's
> +     * split a huge page to 4k chunks */
> +    if (target_page_bits != TARGET_PAGE_BITS)
> +       ctx->raddr |= (ctx->eaddr & (( 1 << target_page_bits ) - 1))
> + & TARGET_PAGE_MASK;
>      return ret;
>  }
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson