Re: [PATCH v1 3/5] xen/riscv: introduce setup_mm()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.10.2024 16:27, oleksii.kurochko@xxxxxxxxx wrote:
> On Thu, 2024-10-17 at 17:15 +0200, Jan Beulich wrote:
>> On 16.10.2024 11:15, Oleksii Kurochko wrote:
>>> +vaddr_t directmap_virt_end __read_mostly;
>>
>> __ro_after_init? And moved ahead of the identifier, just like ...
>>
>>> +/*
>>> + * Setup memory management
>>> + *
>>> + * RISC-V 64 has a large virtual address space (the minimum
>>> supported
>>> + * MMU mode is Sv39, which provides TBs of VA space).
>>> + * In the case of RISC-V 64, the directmap and frametable are
>>> mapped
>>> + * starting from physical address 0 to simplify the page_to_mfn(),
>>> + * mfn_to_page(), and maddr_to_virt() calculations, as there is no
>>> need
>>> + * to account for {directmap, frametable}_base_pdx in this setup.
>>> + */
>>> +void __init setup_mm(void)
>>
>> ... __init is placed e.g. here.
>>
>> It's also unclear why the variable needs to be non-static.
> As it could be use then for some ASSERT(), for example, in
> virt_to_page() as Arm or x86 do.

"Could be" is too little here. Misra dislikes identifiers which could
be static, but aren't. If it's not used right now (nor any time soon),
it should imo be static until a use appears requiring it to be globally
visible.

>>> +{
>>> +    const struct membanks *banks = bootinfo_get_mem();
>>> +    paddr_t ram_end = 0;
>>> +    paddr_t ram_size = 0;
>>> +    unsigned int i;
>>> +
>>> +    /*
>>> +     * We need some memory to allocate the page-tables used for
>>> the directmap
>>> +     * mappings. But some regions may contain memory already
>>> allocated
>>> +     * for other uses (e.g. modules, reserved-memory...).
>>> +     *
>>> +     * For simplicity, add all the free regions in the boot
>>> allocator.
>>> +     */
>>> +    populate_boot_allocator();
>>> +
>>> +    total_pages = 0;
>>> +
>>> +    for ( i = 0; i < banks->nr_banks; i++ )
>>> +    {
>>> +        const struct membank *bank = &banks->bank[i];
>>> +        paddr_t bank_end = bank->start + bank->size;
>>> +
>>> +        ram_size = ram_size + bank->size;
>>> +        ram_end = max(ram_end, bank_end);
>>> +    }
>>> +
>>> +    setup_directmap_mappings(PFN_DOWN(ram_end));
>>
>> While you may want to use non-offset-ed mappings, I can't see how you
>> can
>> validly map just a single huge chunk, no matter whether there are
>> holes
>> in there. Such holes could hold MMIO regions, which I'm sure would
>> require
>> more careful mapping (to avoid cacheable accesses, or even
>> speculative
>> ones).
> My intention was to avoid subraction of directmap_start ( = ram_start )
> in maddr_to_virt() to have less  operations during maddr to virt
> calculation:
>    static inline void *maddr_to_virt(paddr_t ma)
>    {
>        /* Offset in the direct map, accounting for pdx compression */
>        unsigned long va_offset = maddr_to_directmapoff(ma);
>    
>        ASSERT(va_offset < DIRECTMAP_SIZE);
>    
>        return (void *)(DIRECTMAP_VIRT_START /* - directmap_start */ +
>    va_offset);
>    }
> But it seems I don't have any chance to avoid that because of mentioned
> by you reasons... Except probably to have a config which will hard code
> RAM_START for each platform ( what on other hand will make Xen less
> flexible in some point ).
> Does it make sense to have a config instead of identifying ram_start in
> runtime?

I don't think building Xen for just one (kind of) platform is going to be
overly useful (except maybe for development purposes, yet the goal ought
to be to be flexible, and hence it may be a bad idea even while developing
new code).

Jan