Re: [PATCH v1] xen/riscv: Increase XEN_VIRT_SIZE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.04.2025 09:31, Oleksii Kurochko wrote:
> 
> On 4/4/25 8:56 AM, Jan Beulich wrote:
>> On 03.04.2025 18:20, Oleksii Kurochko wrote:
>>> On 4/1/25 6:04 PM, Jan Beulich wrote:
>>>> On 01.04.2025 17:58, Oleksii Kurochko wrote:
>>>>> On 3/31/25 6:14 PM, Jan Beulich wrote:
>>>>>> On 31.03.2025 17:20, Oleksii Kurochko wrote:
>>>>>>> +        _AC(XEN_VIRT_START, UL) >> vpn1_shift;
>>>>>>> +    const unsigned long xen_virt_end_vpn =
>>>>>>> +        xen_virt_starn_vpn + ((XEN_VIRT_SIZE >> vpn1_shift) - 1);
>>>>>>> +
>>>>>>>         if ((va >= DIRECTMAP_VIRT_START) &&
>>>>>>>             (va <= DIRECTMAP_VIRT_END))
>>>>>>>             return directmapoff_to_maddr(va - directmap_virt_start);
>>>>>>>     
>>>>>>> -    BUILD_BUG_ON(XEN_VIRT_SIZE != MB(2));
>>>>>>> -    ASSERT((va >> (PAGETABLE_ORDER + PAGE_SHIFT)) ==
>>>>>>> -           (_AC(XEN_VIRT_START, UL) >> (PAGETABLE_ORDER + 
>>>>>>> PAGE_SHIFT)));
>>>>>>> +    BUILD_BUG_ON(XEN_VIRT_SIZE != MB(8));
>>>>>> Is it necessary to be != ? Won't > suffice?
>>>>> It could be just > MB(2). Or perphaps >=.
>>>>> = would make the build fail, wouldn't it?
>>> I just realized that BUILD_BUG_ON() condition is compared to zero so 
>>> actually everything what
>>> will make the condition true will cause a build fail as inside it used 
>>> !(condition).
>> ???
> 
> |BUILD_BUG_ON()| forces a compilation error if the given condition is true. 
> Therefore, if the condition
> |XEN_VIRT_SIZE != MB(2)| is changed to|XEN_VIRT_SIZE > MB(2)|, the condition 
> will always evaluate to true
> (assuming|XEN_VIRT_SIZE| is greater than 2 MB), which will result in a 
> compilation error.

Well, it was you who used MB(2) in a reply, when previously talk was of MB(8),
and that to grow to MB(16). The BUILD_BUG_ON() is - aiui - about you having set
aside enough page table space. Quite possibly the need for this BUILD_BUG_ON()
then disappears altogether when XEN_VIRT_SIZE is properly taken into account
for the number-of-page-tables calculation. In no event do I see why the MB(2)
boundary would be relevant for anything going forward.

Jan