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

[Andy Lutomirski]

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?

--Andy