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Re: [PATCH v3 5/6] xen/arm: mpu: Enable MPU
- To: Ayan Kumar Halder <ayankuma@xxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Mon, 21 Oct 2024 16:24:41 +0000
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- Cc: Julien Grall <julien@xxxxxxx>, Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Mon, 21 Oct 2024 16:25:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v3 5/6] xen/arm: mpu: Enable MPU
Hi Ayan,
>>> + */
>>> +FUNC_LOCAL(enable_mpu)
>>> + mrs x0, SCTLR_EL2
>>> + bic x0, x0, #SCTLR_ELx_BR /* Disable Background region */
>>> + orr x0, x0, #SCTLR_Axx_ELx_M /* Enable MPU */
>>> + orr x0, x0, #SCTLR_Axx_ELx_C /* Enable D-cache */
>>> + orr x0, x0, #SCTLR_Axx_ELx_WXN /* Enable WXN */
>>
>> NIT: Can't we have a single "orr" instruction to set all the flags?
> Yes, I will change this.
>>
>>> + dsb sy
>>
>> I still question this use of "dsb sy"...
>
> Actually I kept this to ensure that all outstanding memory access are
> completed before MPU is enabled.
>
> However, prepare_xen_region() is invoked before this and it has a 'dsb sy' at
> the end.
>
> So we can drop this barrier.
I suggest to keep the barrier here and drop the one in prepare_xen_region
instead,
have a look in my branch:
https://gitlab.com/xen-project/xen/-/merge_requests/7/diffs?commit_id=f42a2816f9bd95f2f6957887be910cb9acd140b5
During my testing I was having trouble without this barrier.
Cheers,
Luca
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