Re: [PATCH v2 22/24] xen/arm: mm: Embed permission in the flags

[Julien Grall <julien.grall@arm.com>]

On 20/09/2017 22:07, Stefano Stabellini wrote:
> On Wed, 20 Sep 2017, Julien Grall wrote:
>> Hi,
>>
>> On 20/09/17 00:59, Stefano Stabellini wrote:
>>> On Tue, 12 Sep 2017, Julien Grall wrote:
>>>> Currently, it is not possible to specify the permission of a new
>>>> mapping. It would be necessary to use the function modify_xen_mappings
>>>> with a different set of flags.
>>>>
>>>> Introduce a couple of new flags for the permissions (Non-eXecutable,
>>>> Read-Only) and also provides definition that combine the memory attribute
>>>> and permission for common combinations.
>>>>
>>>> PAGE_HYPERVISOR is now an alias to PAGE_HYPERVISOR_RW (read-write,
>>>> non-executable mappings). This does not affect the current mapping using
>>>> PAGE_HYPERVISOR because Xen is currently forcing all the mapping to be
>>>> non-executable by default (see mfn_to_xen_entry).
>>>>
>>>> A follow-up patch will change modify_xen_mappings to use the new flags.
>>>>
>>>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>>>
>>>> ---
>>>>
>>>>      Changes in v2:
>>>>          - Update the commit message
>>>> ---
>>>>   xen/include/asm-arm/page.h | 22 +++++++++++++++++++---
>>>>   1 file changed, 19 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
>>>> index 4022b7dc33..814ed126ec 100644
>>>> --- a/xen/include/asm-arm/page.h
>>>> +++ b/xen/include/asm-arm/page.h
>>>> @@ -66,12 +66,28 @@
>>>>    * Layout of the flags used for updating the hypervisor page tables
>>>>    *
>>>>    * [0:2] Memory Attribute Index
>>>> + * [3:4] Permission flags
>>>>    */
>>>>   #define PAGE_AI_MASK(x) ((x) & 0x7U)
>>>>   -#define PAGE_HYPERVISOR         (MT_NORMAL)
>>>> -#define PAGE_HYPERVISOR_NOCACHE (MT_DEVICE_nGnRE)
>>>> -#define PAGE_HYPERVISOR_WC      (MT_NORMAL_NC)
>>>> +#define _PAGE_XN_BIT    3
>>>> +#define _PAGE_RO_BIT    4
>>>> +#define _PAGE_XN    (1U << _PAGE_XN_BIT)
>>>> +#define _PAGE_RO    (1U << _PAGE_RO_BIT)
>>>> +#define PAGE_XN_MASK(x) (((x) >> _PAGE_XN_BIT) & 0x1U)
>>>> +#define PAGE_RO_MASK(x) (((x) >> _PAGE_RO_BIT) & 0x1U)
>>>> +
>>>> +/* Device memory will always be mapped read-write non-executable. */
>>>> +#define _PAGE_DEVICE    _PAGE_XN
>>>> +#define _PAGE_NORMAL    MT_NORMAL
>>>
>>> I think I understand the intent behind these two definitions, but I find
>>> them more confusing then useful. Specifically, I find confusing that
>>> _PAGE_DEVICE specifies permissions but not memory attributes, while
>>> _PAGE_NORMAL specifies memory attributes but not permissions.
>>
>> Well, it is just contain the common bits for normal memory and device memory.
>> They are not related and are not meant to be used outside of this file except
>> for very specific use case.
>
> Yes, I think that is key. More below.
>
>
>> Such as you want to introduce a new device type
>> and you want to default attributes. Hence the prefixed underscore.
>>
>> Furthermore, it is much easier to reason with _PAGE_DEVICE rather than
>> _PAGE_XN. At least you have one place explaining why the mapping is
>> non-executable. And also it also extending default attribute more easily.
>
> If they are not mean to be used outside of this file, then I am fine with
> them. But please write it in the comment explicitly on top for them.
>
> Something like:
>
> * Convenience #defines to build the PAGE_HYPERVISOR* defines below. Not
> * meant to be used outside of this file.

I will add that.

Cheers,

-- 
Julien Grall

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