On Thu, Dec 15, 2016 at 8:44 AM, Thomas Gleixner <[email protected]> wrote:
> Provide and use a toggle helper instead of doing it with a branch.
>
> x86_64:
> 3662 8505 16 12183 2f97 Before
> 3646 8505 16 12167 2f87 After
>
> i386:
> 5906 9388 1804 17098 42ca Before
> 5834 9324 1740 16898 4202 After
>
> Signed-off-by: Thomas Gleixner <[email protected]>
> ---
> arch/x86/include/asm/tlbflush.h | 10 ++++++++++
> arch/x86/kernel/process.c | 22 ++++------------------
> 2 files changed, 14 insertions(+), 18 deletions(-)
>
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -110,6 +110,16 @@ static inline void cr4_clear_bits(unsign
> }
> }
>
> +static inline void cr4_toggle_bits(unsigned long mask)
> +{
> + unsigned long cr4;
> +
> + cr4 = this_cpu_read(cpu_tlbstate.cr4);
> + cr4 ^= mask;
> + this_cpu_write(cpu_tlbstate.cr4, cr4);
> + __write_cr4(cr4);
> +}
This scares me for the same reason as BTF, although this should at
least be less fragile. But how about:
static inline void cr4_set_bit_to(unsigned long mask, bool set)
{
...
cr4 &= ~mask;
cr4 ^= (set << ilog2(mask));
...
}
This should generate code that's nearly as good.
