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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

>> And why is this not CONFIG_HAS_VMAP anyway (with HAS_VMAP
>> properly moved to common/Kconfig, where e.g. HAS_PMAP also lives,
>> and then unconditionally selected by all other architectures)?
> 
> I am not sure why CONFIG_HAS_VMAP should be moved to common/Kconfig when 
> it will be selected/deselected only for Arm architecture.

Because you will want to use that (not MMU) in vm_init().

Jan