[Xen-devel] [RESEND] Question about recursive mappings

[Mathieu Ropert <mro@xxxxxxxxxx>]

[Previous one didn't hit list after 3 days, trying a resend, sorry if 
both finally show up]
Hi,

are recursive mappings (ie: a page table entry pointing back to itself)
supported by Xen (on x86_64 at least)?
I'm asking cause i'm seeing many error logs from get_page_type() telling
something like "saw L3_page_table expected L2_page_table" or "saw
L4_page_table expected L3_page_table" (finally leading to a failing 
mmu_update, i guess others happens on user pagetables switches).
Or maybe is there any workaround needed? (I think i saw something like
setting entry to 0 first then to the recursive entry somewhere, but
can't remember where).
By the way, i'm using recursive mappings in kernel page directory (which
seems ok) and i temporay make user page directory recursive when i map
a user PGD in kernel space (mapping user PGD to a L4 entry of kernel 
tables, then using kernel L4 slot and user PGD recursive entry to access 
user page tables).
[edit]
Done some little research about the problem. Seems like NetBSD use the 
same thing and works, but there is no x86_64 ports for now. I'm starting 
to think that may be a x86_64 issue, maybe because recursive mappings 
don't lead to conflicting types with only 2 levels.
Xen interface states that a page can only be of one type (PGD, PT, LDT, 
GDT  and R/W). I don't know why there is a need to distinguish page 
table levels, but i'm afraid this restriction will conflit with some MMU 
implementation on x86_64 like NetBSD and OpenBSD, and maybe others 
(FreeBSD on top of my mind, don't know how much the pmap implementation 
diverged).
[/edit]

Regards,

Mathieu


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