[U-Boot] [PATCH v9 12/18] sandbox: Enhance map_to_sysmem() to handle foreign pointers

[Simon Glass]

Hi Alex,

On 15 September 2018 at 10:16, Alexander Graf <agraf at suse.de> wrote:
>
>
> On 14.09.18 17:46, Simon Glass wrote:
>> Hi Alex,
>>
>> On 26 August 2018 at 19:11, Alexander Graf <agraf at suse.de> wrote:
>>>
>>>
>>> On 08.08.18 11:54, Simon Glass wrote:
>>>> At present map_sysmem() maps an address into the sandbox RAM buffer,
>>>> return a pointer, while map_to_sysmem() goes the other way.
>>>>
>>>> The mapping is currently just 1:1 since a case was not found where a more
>>>> flexible mapping was needed. PCI does have a separate and more complex
>>>> mapping, but uses its own mechanism.
>>>>
>>>> However this arrange cannot handle one important case, which is where a
>>>> test declares a stack variable and passes a pointer to it into a U-Boot
>>>> function which uses map_to_sysmem() to turn it into a address. Since the
>>>> pointer is not inside emulated DRAM, this will fail.
>>>>
>>>> Add a mapping feature which can handle any such pointer, mapping it to a
>>>> simple tag value which can be passed around in U-Boot as an address.
>>>>
>>>> Signed-off-by: Simon Glass <sjg at chromium.org>
>>>
>>> I think you are aware that this logic will fall apart spectacularly if
>>> any arithmetic operation happens on the virtual (U-Boot) address, right?
>>> So simple code like
>>>
>>>   readl(vaddr + 1);
>>>
>>> will just fail (hopefully) or (more likely) return a completely
>>> incorrect value.
>>>
>>> I assume this is intentional, but shouldn't the tag increment be
>>> something slightly larger then?
>>
>> What do you expect readl() to do on sandbox? At present it is just a
>> no-op. I suppose we could support memory-mapped I/O but it has not
>> been attempted yet.
>
> It was really just meant as an arbitrary example of something where you
> assume "address + 1" == "pointer(address) + 1".

So long as the addresses are within the range sandbox supports
(normally 0 to 128MB unless you increase RAM size) then there is no
problem adding 1 to either an address (or a pointer derived from it
with map_sysmem()).

If you use an address outside that range, it is actually a tag, not an
address. Trying to use an out-of-range address is invalid anyway, so
it probably doesn't matter.

Regards,
Simon