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Re: [PATCH V3] xen/dom0less: Calculate guest DTB size based on MAX_VIRT_CPUS
- To: Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: "Orzel, Michal" <michal.orzel@xxxxxxx>
- Date: Thu, 18 Dec 2025 13:50:44 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Grygorii Strashko <grygorii_strashko@xxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Harry Ramsey <harry.ramsey@xxxxxxx>
- Delivery-date: Thu, 18 Dec 2025 12:51:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18/12/2025 08:44, Oleksandr Tyshchenko wrote:
>
>
> On 18.12.25 02:05, Stefano Stabellini wrote:
>
> Hello Stefano
>
>> On Wed, 17 Dec 2025, Oleksandr Tyshchenko wrote:
>>> Creating a dom0less guest with a high vCPU count (e.g., >32) fails
>>> because the fixed 4KiB device tree buffer (DOMU_DTB_SIZE) overflows
>>> during creation.
>>>
>>> The FDT nodes for each vCPU are the primary consumer of space,
>>> and the previous fixed-size buffer was insufficient.
>>>
>>> This patch replaces the fixed size with a formula that calculates
>>> the required buffer size based on a fixed baseline plus a scalable
>>> portion for each potential vCPU up to the MAX_VIRT_CPUS limit.
>>>
>>> Please note, the change to DOMU_DTB_SIZE formula would result in
>>> a smaller buffer size of 3072 bytes compared to the original 4096 bytes
>>> on Arm32 platforms where MAX_VIRT_CPUS is 8.
>>
>> I am OK with this patch I would only ask to retain the minimum size of
>> 4KB due to the possible presence of passthrough device nodes.
>
> I think there might be cases when even 4KB would not be enough to cover
> the whole dtb with passthrough device nodes. But the existing code
> should already handle that, so if a partial device tree is provided,
> then it will be accounted separately:
>
> /* Account for domU passthrough DT size */
> if ( kinfo->dtb )
> fdt_size += kinfo->dtb->size;
I agree with Oleksandr:
Acked-by: Michal Orzel <michal.orzel@xxxxxxx>
~Michal
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