Re: Mapping an executable page

[Timur Tabi <timur@freescale.com>]

Thomas De Schampheleire wrote:

> * However, if you jump to an address in that page, you'll have to make
> sure that the entire code that executes is mapped (make map_size large
> enough).

Well, that seems obvious.

> * When that range spanned multiple pages, I faced the issue of only
> one page being actually mapped in the TLBs. My assumption is that the
> call to __ioremap not necessarily updates the TLBs, but mainly some
> kernel-internal tables. The actual TLB mapping presumably happens when
> a data exception occurs.

Hmmm.... I find that surprising.  Memory allocated via ioremap() is supposed to
be available in interrupt handlers, where TLB mappings can't be created
on-the-fly.  I'm not sure that your observation is correct.

> * Therefore, to make sure that the mapping I intended with __ioremap()
> is actually reflected in the TLB tables, I added dummy reads of each
> page in the TLB, prior to jumping to the boot code, as follows:
>                 /* make sure memory is read, once every 4Kbyte is enough */
>                 for (p = vaddr; p < vaddr + map_size; p += 0x1000) {

You should at least use PAGE_SIZE instead of 0x1000.

>                         unsigned long dummy = *(volatile unsigned long *)p;
>                         (void)dummy;
>                 }
> 
> * After these changes (make sure all code is mapped + make sure to
> read all pages so that the TLBs are updated), my scenario works fine.

I still find it hard to believe that this is necessary.

-- 
Timur Tabi
Linux kernel developer at Freescale