linux-kernel - Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Message-ID: <YxLXJk36EKxldC1S@nazgul.tnic>
Date:   Sat, 3 Sep 2022 06:25:36 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
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Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

On Fri, Sep 02, 2022 at 03:33:20PM +0000, Kalra, Ashish wrote:
> Yes we want to map the faulting address to a RMP entry, but hugepage
> entries in RMP table are basically subpage 4K entries. So it is a 4K
> entry when the page is a 2M one and also a 4K entry when the page is a
> 1G one.

Wait, what?!

APM v2 section "15.36.11 Large Page Management" and PSMASH are then for
what exactly?

> That's why the computation to get a 4K page index within a 2M/1G
> hugepage mapping is required.

What if a guest RMP-faults on a 2M page and there's a corresponding 2M
RMP entry? What do you need the 4K entry then for?

Hell, __snp_lookup_rmpentry() even tries to return the proper page
level...

/me looks in disbelief in your direction...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette