Re: [PATCH 8/9] x86/mm: Make __VIRTUAL_MASK_SHIFT dynamic
From: Kirill A. Shutemov
Date: Wed Feb 14 2018 - 12:55:57 EST
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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > ---
> > 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