[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 11/40] xen/mpu: build up start-of-day Xen MPU memory region map
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 theprocess 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
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