Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table

[Borislav Petkov]

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.

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