Re: [PATCH 8/9] x86/mm: Make __VIRTUAL_MASK_SHIFT dynamic

[Andy Lutomirski]

On Wed, Feb 14, 2018 at 5:55 PM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> 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
>

Nice!