linux-kernel - Re: [PATCH v3 1/2] x86/mm/KASLR: Only build one PUD entry of area for real mode trampoline

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Message-ID: <20190312073849.d6xezoaaffmbeyx5@kshutemo-mobl1>
Date:   Tue, 12 Mar 2019 10:38:49 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, keescook@...omium.org, thgarnie@...gle.com
Subject: Re: [PATCH v3 1/2] x86/mm/KASLR: Only build one PUD entry of area
 for real mode trampoline

On Fri, Mar 08, 2019 at 10:56:15AM +0800, Baoquan He wrote:
> The current code builds identity mapping for real mode treampoline by
> borrowing page tables from the direct mapping section if KASLR is
> enabled. It will copy present entries of the first PUD table in 4-level
> paging mode, or the first P4D table in 5-level paging mode.
> 
> However, there's only a very small area under low 1 MB reserved
> for real mode trampoline in reserve_real_mode(). Makes no sense
> to build up so large area of mapping for it. Since the randomization
> granularity in 4-level is 1 GB, and 512 GB in 5-level, only copying
> one PUD entry is enough.
> 
> Hence, only copy one PUD entry of area where physical address 0
> resides. And this is preparation for later changing the randomization
> granularity of 5-level paging mode from 512 GB to 1 GB.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov