Re: [PATCH v2 2/4] xen: arm: make VMAP only support in MMU system

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.08.2024 18:02, Ayan Kumar Halder wrote:
> Hi Jan,
> 
> On 09/08/2024 10:34, Jan Beulich wrote:
>> On 08.08.2024 17:50, Ayan Kumar Halder wrote:
>>> On 08/08/2024 13:49, Jan Beulich wrote:
>>>> On 08.08.2024 14:09, Ayan Kumar Halder wrote:
>>>>> @@ -58,9 +58,13 @@ config PADDR_BITS
>>>>>           default 40 if ARM_PA_BITS_40
>>>>>           default 48 if ARM_64
>>>>>    
>>>>> +config HAS_VMAP
>>>>> + def_bool y
>>>> With this being always enabled, ...
>>> I had to define the config somewhere. It seemed this is the correct
>>> place to define as HAS_VMAP will be turned off when MPU is introduced.
>>>
>>> So, it will be
>>>
>>> config HAS_VMAP
>>>
>>>              def_bool n
>>>
>>> At that time, only MMU will select HAS_VMAP.
>> Well, but why is it not simply
>>
>> config HAS_VMAP
>>      bool
>>
>> from the very beginning? (There should never be "def_bool n" imo, btw.)
>>
>>>>> --- a/xen/include/xen/vmap.h
>>>>> +++ b/xen/include/xen/vmap.h
>>>>> @@ -141,7 +141,9 @@ void *arch_vmap_virt_end(void);
>>>>>    /* Initialises the VMAP_DEFAULT virtual range */
>>>>>    static inline void vm_init(void)
>>>>>    {
>>>>> +#ifdef CONFIG_MMU
>>>>>        vm_init_type(VMAP_DEFAULT, (void *)VMAP_VIRT_START, 
>>>>> arch_vmap_virt_end());
>>>>> +#endif
>>>>>    }
>>>> What about non-Arm, which all have MMUs but no corresponding Kconfig
>>>> setting?
>>> AFAICS , the only file that is of our concern xen/common/vmap.c It is
>>> enclosed with VMAP_VIRT_START which is defined in mmu specific file
>>> (xen/arch/arm/include/asm/mmu/layout.h).
>>>
>>> So, it will not be compiled for Arm MPU arch.
>> Yet that wasn't my question. I asked about non-Arm, for all of which it
>> looks like you're changing behavior of vm_init() (by suddenly making it
>> do nothing).
> 
> Oh now I see what you mean. There is no CONFIG_MMU is x86 , yet it is 
> used to enclose a common code which is a mistake.
> 
> So, we should introduce HAS_VMAP in xen/common/Kconfig and select it 
> from X86, X86_64 and Arm.
> 
> Is my understanding correct ?

Almost. On Arm the select would be conditional, depending on (or simply
from) MMU.

Jan