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

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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On 4/4/25 9:52 AM, Jan Beulich wrote:
> 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.

Also, doesn’t|BUILD_BUG_ON()| affect how the|ASSERT()| that follows it is written?

The changes, at the moment, look like:
+    const unsigned int vpn1_shift = PAGETABLE_ORDER + PAGE_SHIFT;
+    const unsigned long va_vpn = va >> vpn1_shift;
+    const unsigned long xen_virt_start_vpn =
+        _AC(XEN_VIRT_START, UL) >> vpn1_shift;
+    const unsigned long xen_virt_end_vpn =
+        xen_virt_start_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(16));
+    ASSERT((va_vpn >= xen_virt_start_vpn) && (va_vpn <= xen_virt_end_vpn));


If|XEN_VIRT_SIZE| is greater than|GB(1)|, then|xen_virt_end_vpn| may be calculated
incorrectly.

For example, if|XEN_VIRT_START| is|0xFFFFFFFF80000000| and|XEN_VIRT_SIZE| is|0x40200000|,
then|(XEN_VIRT_SIZE >> vpn1_shift)| equals 513, whereas|va_vpn| is always in the range [0, 511],
but xen_virt_end_vpn will be greater then 511.

So shouldn't it  be checked before ASSERT() that XEN_VIRT_SIZE is <= GB(1):
   BUILD_BUG_ON(XEN_VIRT_SIZE <= GB(1))?

~ Oleksii


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