Re: [PATCH] x86/irq: Improve local_irq_restore() code generation and performance

[Andrew Cooper <amc96@xxxxxxxx>]

On 06/12/2021 13:38, Andrew Cooper wrote:
> popf is a horribly expensive instruction, while sti is an optimised fastpath.
> Switching popf for a conditional branch and sti caused an 8% perf improvement
> in various linux measurements.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> ---
>  xen/include/asm-x86/system.h | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
> index 65e63de69a67..4be235472ecd 100644
> --- a/xen/include/asm-x86/system.h
> +++ b/xen/include/asm-x86/system.h
> @@ -267,13 +267,8 @@ static inline unsigned long 
> array_index_mask_nospec(unsigned long index,
>  })
>  #define local_irq_restore(x)                                     \
>  ({                                                               \
> -    BUILD_BUG_ON(sizeof(x) != sizeof(long));                     \
> -    asm volatile ( "pushfq\n\t"                                  \
> -                   "andq %0, (%%rsp)\n\t"                        \
> -                   "orq  %1, (%%rsp)\n\t"                        \
> -                   "popfq"                                       \
> -                   : : "i?r" ( ~X86_EFLAGS_IF ),                 \
> -                       "ri" ( (x) & X86_EFLAGS_IF ) );           \
> +    if ( (x) & X86_EFLAGS_IF )                                   \
> +        local_irq_enable();                                      \
>  })

Bah.  There's still the one total abuse of local_irq_restore() to
disable interrupts.

I'll do a pre-requisite patch.

~Andrew