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Re: [PATCH 4/6] arm/mpu: Destroy an existing entry in Xen MPU memory mapping table
- To: Ayan Kumar Halder <ayankuma@xxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Wed, 2 Jul 2025 13:44:08 +0000
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- Cc: Hari Limaye <Hari.Limaye@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Penny Zheng <Penny.Zheng@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Wei Chen <Wei.Chen@xxxxxxx>
- Delivery-date: Wed, 02 Jul 2025 13:44:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-index: AQHb4cpciV/6CvSAB0CHD6KSA8R6qrQUHtAAgAlF/ZmAAX0kgIAACRmA
- Thread-topic: [PATCH 4/6] arm/mpu: Destroy an existing entry in Xen MPU memory mapping table
Hi Ayan,
> On 2 Jul 2025, at 14:11, Ayan Kumar Halder <ayankuma@xxxxxxx> wrote:
>
>
> On 01/07/2025 15:56, Hari Limaye wrote:
>>
>> Hi Ayan,
>>
> Hi Hari,
>>
>> Thank you for the review. I have just a couple of clarifications before I
>>
>> re-spin the series to address all the comments:
>>
>> > > - if ( flags & _PAGE_PRESENT )
>>
>> > > + if ( (flags & _PAGE_PRESENT) && (MPUMAP_REGION_NOTFOUND == rc) )
>>
>> >
>>
>> > Same question in this patch , why do we need to check for _PAGE_PRESENT.
>>
>> > Can't we just rely on MPUMAP_REGION_XXX ?
>>
>> The _PAGE_PRESENTflag indicates intent - whether the caller intends to create
>>
>> or remove a region.
>>
> If so, then I misunderstood the code. However, looking at
> xen_pt_check_entry(), it seems _PAGE_PRESENTindicates if the page table entry
> exists. If so, using _PAGE_PRESENTis not making sense to me atleast. May be
> others can chime in.
But it seems to me that _PAGE_PRESENT is used in the MPU code in the same way
as the MMU code, to check
if the caller has intention to add/modify a region if it’s set, otherwise to
delete it.
I’m not sure why you say it’s different, can you point out which line in case,
because I’ve had a look on xen_pt_check_entry
but I didn’t get your point.
Cheers,
Luca
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