Messages in this thread | | | Date | Tue, 21 May 2024 13:57:20 +0200 | From | Oscar Salvador <> | Subject | Re: [RFC PATCH v2 01/20] mm: Provide pagesize to pmd_populate() |
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On Mon, May 20, 2024 at 04:24:51PM +0000, Christophe Leroy wrote: > I had a quick look at that document and it seems to provide a good > summary of MMU features and principles. However there are some > theoritical information which is not fully right in practice. For > instance when they say "Segment attributes. These fields define > attributes common to all pages in this segment.". This is right in > theory if you consider it from Linux page table topology point of view, > hence what they call a segment is a PMD entry for Linux. However, in > practice each page has its own L1 and L2 attributes and there is not > requirement at HW level to have all L1 attributes of all pages of a > segment the same.
Thanks for taking the time Christophe, highly appreciated.
> rlwimi = Rotate Left Word Immediate then Mask Insert. Here it rotates > r10 by 23 bits to the left (or 9 to the right) then masks with > _PMD_PAGE_512K and inserts it into r11. > > It means _PAGE_HUGE bit is copied into lower bit of PS attribute. > > PS takes the following values: > > PS = 00 ==> Small page (4k or 16k) > PS = 01 ==> 512k page > PS = 10 ==> Undefined > PS = 11 ==> 8M page
I see, thanks for the explanation.
> That's a RFC, all ideas are welcome, I needed something to replace > hugepd_populate()
The only user interested in pmd_populate() having a sz parameter is 8xx because it will toggle _PMD_PAGE_8M in case of a 8MB mapping.
Would it be possible for 8xx to encode the 'sz' in the *pmd pointer prior to calling down the chain? (something like as we do for PTR_ERR()). Then pmd_populate_{kernel_}size() from 8xx, would extract it like:
unsigned long sz = PTR_SIZE(pmd)
Then we would not need all these 'sz' parameters scattered.
Can that work?
PD: Do you know a way to emulate a 8xx VM? qemu seems to not have support support.
Thanks
-- Oscar Salvador SUSE Labs
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