Messages in this thread | | | Date | Mon, 19 Apr 2021 14:32:26 +0200 | From | Borislav Petkov <> | Subject | Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table |
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On Wed, Mar 24, 2021 at 12:04:10PM -0500, Brijesh Singh wrote: > A write from the hypervisor goes through the RMP checks. When the > hypervisor writes to pages, hardware checks to ensures that the assigned > bit in the RMP is zero (i.e page is shared). If the page table entry that > gives the sPA indicates that the target page size is a large page, then > all RMP entries for the 4KB constituting pages of the target must have the > assigned bit 0.
Hmm, so this is important: I read this such that we can have a 2M page table entry but the RMP table can contain 4K entries for the corresponding 512 4K pages. Is that correct?
If so, then there's a certain discrepancy here and I'd expect that if the page gets split/collapsed, depending on the result, the RMP table should be updated too, so that it remains in sync.
For example:
* mm decides to group all 512 4K entries into a 2M entry, RMP table gets updated in the end to reflect that
* mm decides to split a page, RMP table gets updated too, for the same reason.
In this way, RMP table will be always in sync with the pagetables.
I know, I probably am missing something but that makes most sense to me instead of noticing the discrepancy and getting to work then, when handling the RMP violation.
Or?
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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