linux-kernel - Re: [PATCH 7/8] x86: Enable 5-level paging support

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Date:   Thu, 6 Apr 2017 18:24:38 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Juergen Gross <jgross@...e.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/8] x86: Enable 5-level paging support

On Thu, Apr 06, 2017 at 04:52:11PM +0200, Juergen Gross wrote:
> On 06/04/17 16:01, Kirill A. Shutemov wrote:
> > Most of things are in place and we can enable support of 5-level paging.
> > 
> > Enabling XEN with 5-level paging requires more work. The patch makes XEN
> > dependent on !X86_5LEVEL.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/Kconfig     | 5 +++++
> >  arch/x86/xen/Kconfig | 1 +
> >  2 files changed, 6 insertions(+)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 4e153e93273f..7a76dcac357e 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -318,6 +318,7 @@ config FIX_EARLYCON_MEM
> >  
> >  config PGTABLE_LEVELS
> >  	int
> > +	default 5 if X86_5LEVEL
> >  	default 4 if X86_64
> >  	default 3 if X86_PAE
> >  	default 2
> > @@ -1390,6 +1391,10 @@ config X86_PAE
> >  	  has the cost of more pagetable lookup overhead, and also
> >  	  consumes more pagetable space per process.
> >  
> > +config X86_5LEVEL
> > +	bool "Enable 5-level page tables support"
> > +	depends on X86_64
> > +
> >  config ARCH_PHYS_ADDR_T_64BIT
> >  	def_bool y
> >  	depends on X86_64 || X86_PAE
> > diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
> > index 76b6dbd627df..b90d481ce5a1 100644
> > --- a/arch/x86/xen/Kconfig
> > +++ b/arch/x86/xen/Kconfig
> > @@ -5,6 +5,7 @@
> >  config XEN
> >  	bool "Xen guest support"
> >  	depends on PARAVIRT
> > +	depends on !X86_5LEVEL
> >  	select PARAVIRT_CLOCK
> >  	select XEN_HAVE_PVMMU
> >  	select XEN_HAVE_VPMU
> > 
> 
> Just a heads up: this last change will conflict with the Xen tree.

It should be trivial to fix, right? It's one-liner after all.

> Can't we just ignore the additional level in Xen pv mode and run with
> 4 levels instead?

We don't have yet boot-time switching between paging modes yet. It will
come later. So the answer is no.

-- 
 Kirill A. Shutemov