Re: [PATCH v2 11/40] xen/mpu: build up start-of-day Xen MPU memory region map

[Julien Grall <julien@xen.org>]

Hi Penny,

On 01/02/2023 05:39, Penny Zheng wrote:
>>> If we are adding MPU regions in sequence as you suggested, while using
>>> bitmap at the same time to record used entry.
>>> TBH, this is how I designed at the very beginning internally. We found
>>> that if we don't do reorg late-boot to keep fixed in front and
>>> switching ones after, each time when we do vcpu context switch, not
>>> only we need to hunt down switching ones to disable, while we add new
>>> switch-in regions, using bitmap to find free entry is saying that the
>> process is unpredictable. Uncertainty is what we want to avoid in Armv8-R
>> architecture.
>>
>> I don't understand why it would be unpredictable. For a given combination
>> of platform/device-tree, the bitmap will always look the same. So the
>> number of cycles/instructions will always be the same.
>>
> 
> In boot-time, it will be always the same. But if we still use bitmap to find free
> entry(for switching MPU regions) on runtime, hmmm, I thought this part will
> be unpredictable.

I know this point is now moot as we agreed on not using a bitmap but I 
wanted to answer on the unpredictability part.

It depends on whether you decide to allocate more entry at runtime. My 
assumption is you won't and therefore the the time to walk the bitmap 
will always be consistent.

Cheers,

-- 
Julien Grall