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Re: [PATCH v4 2/4] xen/arm: optimize the size of struct vcpu
- To: "Orzel, Michal" <michal.orzel@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 29 Dec 2025 11:27:27 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Mon, 29 Dec 2025 11:27:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 29/12/2025 11:10 am, Orzel, Michal wrote:
>
> On 29/12/2025 12:08, Orzel, Michal wrote:
>>
>> On 23/12/2025 18:01, Oleksii Kurochko wrote:
>>> When CONFIG_NEW_VGIC=y and CONFIG_ARM_64=y, the size of struct vcpu
>>> exceeds one page, which requires allocating two pages and led to the
>>> introduction of MAX_PAGES_PER_VCPU.
> Also, I think it would be better to drop MAX_PAGES_PER_VCPU in this patch.
Or at least shrink it to just 1, which would be minimal churn.
>
> ~Michal
>
>>> To remove the need for MAX_PAGES_PER_VCPU in a follow-up patch, the vgic
>>> member of NEW_VGIC's struct vgic_vcpu member private_irq is changed to a
>> s/vgic_vcpu/vgic_cpu/
>> s/private_irq/private_irqs/
>>
>>> pointer to struct vgic_irq.
>>> As a result, the size of struct vcpu for Arm64 is reduced to 2176 bytes,
>>> compared to 3840 bytes (without these changes and with CONFIG_ARM_64=y)
>>> and 4736 bytes (without these changes and with both CONFIG_ARM_64=y and
>>> CONFIG_NEW_VGIC=y).
>> You only touch new vGIC, so there should be no size reduction without it but
>> the
>> paragraph reads as if the change affected both old and new vGIC. Also I would
>> mention that probably you provided the numbers based on a defconfig target.
I think that was stale from v1, where this patch was far larger and more
invasive.
~Andrew
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