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Re: [XEN v4] xen/page_alloc: Relax the BUILD_BUG_ON() in xenheap_max_mfn()
- To: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 5 Dec 2022 12:46:17 +0100
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- Cc: sstabellini@xxxxxxxxxx, stefanos@xxxxxxxxxx, julien@xxxxxxx, andrew.cooper3@xxxxxxxxxx, george.dunlap@xxxxxxxxxx, wl@xxxxxxx, bobbyeshleman@xxxxxxxxx
- Delivery-date: Mon, 05 Dec 2022 11:46:25 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05.12.2022 10:48, Ayan Kumar Halder wrote:
> In the near future, we are considering to use a common xen/domain heap for
> Arm 32-bit V8-R. In this setup, both PADDR_BITS and BITS_PER_LONG will be
> 32. Therefore, the condition PADDR_BITS >= BITS_PER_LONG will become true.
>
> Looking at the commit that introduce the BUILD_BUG_ON (88e3ed61642b
> "x86/NUMA: make init_node_heap() respect Xen heap limit") and the current
> use, it seems this check was mainly to ensure that the shift of xenheap_bits
> is not going to be undefined (>> N for a N-bit type is undefined).
>
> So far, all the shifts are using "xenheap_bits - PAGE_SHIFT". Therefore, the
> existing code should work for 32-bit architecture as long as the result of
> the substraction is below 32.
>
> Therefore relax the BUILD_BUG_ON() to check that (PADDR_BITS - PAGE_SHIFT)
> is not equal of above BITS_PER_LONG.
>
> Note that we would need further precaution if we ended up to use
> 'xenheap_bits' alone in shifts.
>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> Signed-off-by: Julien Grall <julien@xxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
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