Re: [PATCH v1] xen/arm: Do not allocate pte entries for MAP_SMALL_PAGES

[Julien Grall <julien.grall@linaro.org>]

On 24/02/15 13:13, Ian Campbell wrote:
>>>>> #define PTE_PRESENT ((struct lpae_t){ .pt.present = 1 }).bits
>>>>>
>>>>> probably doesn't work, I'm not even sure if this sort of thing is
>>>>> possible. If not then "#define PTE_PRESET (1ULL<<0)".
>>>>
>>>> The attribute index (write-alloc, buferrable...) is using the less 
>>>> significant 3 bits. So I was suggesting to use the top of the word.
>>>
>>> I was suggesting to use bits 2..4 as in the real PTE, to be more similar
>>> to the x86 interpretation of this argument.
>>
>> I don't think we have to follow how x86 interpret this argument. This is
>> just a series of flags and may or may not match a bit in the PTE.
> 
> Not matching x86 here has already led to one set of confusion.

That was a misunderstanding of the define. Without Jan's explanation I
would not have understand the purpose of this define.

> I'm not saying with have to match x86, but we should strongly consider
> it and not just run with what we have now because it is a smaller
> change.

lpae_t is an uint64_t and flags an unsigned int, so we will have to
check that any bits we have to modified is effectively living on the
less significant word.

Also, a smaller change would allow us to backport the patch to Xen 4.5.

Regards,

-- 
Julien Grall