Re: [Qemu-devel] [PATCH] PPC: booke206: Check for min/max TLB entry size

[Scott Wood <scottwood@freescale.com>]

On 01/20/2012 07:21 AM, Alexander Graf wrote:
> When setting a TLB entry, we need to check if the TLB we're putting it in
> actually supports the given size. According to the 2.06 PowerPC ISA, a
> value that's out of range results in the minimum page size for the TLB
> to be used.

The ISA says, "If the page size specified by
MAS1TSIZE is not supported by the specified array, the
tlbwe may be performed as if TSIZE were some imple-
mentation-dependent value, or an Illegal Instruction
exception occurs."

In practice, what this means on e500 is that TLB0 (which only supports
one page size) ignores TSIZE.  I'm not sure what happens when you write
an entry to TLB1 with an invalid TSIZE.

> +    /* XXX only applies for MAV 1.0 */
> +    size_tlb = (tlb->mas1 & MAS1_TSIZE_MASK) >> (MAS1_TSIZE_SHIFT + 1);
> +    size_min = (tlbncfg & TLBnCFG_MINSIZE) >> TLBnCFG_MINSIZE_SHIFT;
> +    size_max = (tlbncfg & TLBnCFG_MAXSIZE) >> TLBnCFG_MAXSIZE_SHIFT;
> +    if ((size_tlb > size_max) || (size_tlb < size_min)) {
> +        /* set to min size */
> +        tlb->mas1 &= ~MAS1_TSIZE_MASK;
> +        tlb->mas1 |= size_min << (MAS1_TSIZE_SHIFT + 1);
> +    }

You could just implement a bitmap now, which will work for MAV 2.0 as well.

Especially since we're using the MAV 2.0 definition of tsize already, so
min/max isn't an accurate way to describe what we support.

-Scott