linux-kernel - Re: [PATCH 8/9] x86/mm: Make __VIRTUAL_MASK

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Date:   Wed, 14 Feb 2018 20:55:48 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...e.de>, Andi Kleen <ak@...ux.intel.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] x86/mm: Make __VIRTUAL_MASK_SHIFT dynamic

On Wed, Feb 14, 2018 at 05:22:58PM +0000, Andy Lutomirski wrote:
> On Wed, Feb 14, 2018 at 11:16 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> > For boot-time switching between paging modes, we need to be able to
> > adjust virtual mask shifts.
> >
> > The change doesn't affect the kernel image size much:
> >
> >    text    data     bss     dec     hex filename
> > 8628892 4734340 1368064 14731296         e0c820 vmlinux.before
> > 8628966 4734340 1368064 14731370         e0c86a vmlinux.after
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/entry/entry_64.S            | 12 ++++++++++++
> >  arch/x86/include/asm/page_64_types.h |  2 +-
> >  arch/x86/mm/dump_pagetables.c        | 12 ++++++++++--
> >  arch/x86/mm/kaslr.c                  |  4 +++-
> >  4 files changed, 26 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index cd216c9431e1..1608b13a0b36 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -260,8 +260,20 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
> >          * Change top bits to match most significant bit (47th or 56th bit
> >          * depending on paging mode) in the address.
> >          */
> > +#ifdef CONFIG_X86_5LEVEL
> > +       testl   $1, pgtable_l5_enabled(%rip)
> > +       jz      1f
> > +       shl     $(64 - 57), %rcx
> > +       sar     $(64 - 57), %rcx
> > +       jmp     2f
> > +1:
> > +       shl     $(64 - 48), %rcx
> > +       sar     $(64 - 48), %rcx
> > +2:
> > +#else
> >         shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> >         sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> > +#endif
> 
> Eww.
> 
> Can't this be ALTERNATIVE "shl ... sar ...", "shl ... sar ...",
> X86_FEATURE_5LEVEL or similar?

Optimization comes in a separate patch:

https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/commit/?h=la57/boot-switching/wip&id=015fa3576a7f2b8bd271096bb3a12b06cdc845af

-- 
 Kirill A. Shutemov