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

[Andrew Cooper <amc96@xxxxxxxx>]

On 06/12/2021 14:07, Jan Beulich wrote:
> 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:

These are Linux's APIs, not ours, and they've spoken on the matter. 
Furthermore, I agree with this being an abuse of the mechanism.

>  To me
> "restore" doesn't mean only "maybe re-enable", but also "maybe
> re-disable".

nor does "save" mean "save and disable", but that's what it does.

The naming may not be completely ideal, but the expected usage is very
much one way.

>  And a conditional STI-or-CLI is likely still be better
> than POPF.

It likely is better than popf, but for one single abuse which can be
written in a better way anyway, it's really not worth it.

~Andrew