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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.12.2021 14:55, Andrew Cooper wrote:
> 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.

Question is whether that's really to be considered an abuse: To me
"restore" doesn't mean only "maybe re-enable", but also "maybe
re-disable". And a conditional STI-or-CLI is likely still be better
than POPF.

Jan