linux-kernel - Re: [PATCHv2 1/2] x86/mm: Move LDT remap out of KASLR region on 5-level paging

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Message-ID: <20181025081100.GA31346@MiWiFi-R3L-srv>
Date:   Thu, 25 Oct 2018 16:11:00 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        boris.ostrovsky@...cle.com, jgross@...e.com, willy@...radead.org,
        x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 1/2] x86/mm: Move LDT remap out of KASLR region on
 5-level paging

On 10/25/18 at 10:24am, Kirill A. Shutemov wrote:
> On Thu, Oct 25, 2018 at 10:18:09AM +0800, Baoquan He wrote:
> > > We don't touch 4 pgd slot gap just before the direct mapping reserved
> > > for a hypervisor, but move direct mapping by one slot instead.
> > > 
> > > The LDT mapping is per-mm, so we cannot move it into P4D page table next
> > > to CPU_ENTRY_AREA without complicating PGD table allocation for 5-level
> > > paging.
> > 
> > Here as discussed in private thread, at the first place you also agreed
> > to put it in p4d entry next to CPU_ENTRY_AREA, but finally you changd
> > mind, there must be some reasons when you implemented and investigated
> > further to find out. Could you please say more about how it will
> > complicating PGD table allocation for 5-level paging? Or give an use
> > case where it will complicate?
> 
> On 5-level machine all memory starting from CPU_ENTRY_AREA (and part of
> KASAN memory) is in the same P4D page table. All this memory is shared
> across all processes, we just copy PGD entry -- all proceses point to the
> same P4D page table. (I leave out PTI from the picture for simplicity.)

Yes, got it, I didn't notice this, thanks a lot.