linux-kernel - Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing high-probability event dispatching

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Message-ID: <21a2dc23-a87f-42aa-b5c0-ab828b1c6ad8@zytor.com>
Date: Fri, 17 Jan 2025 08:17:35 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Xin Li <xin@...or.com>, Ethan Zhao <haifeng.zhao@...ux.intel.com>,
        Ethan Zhao <etzhao@...look.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc: tglx@...utronix.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        andrew.cooper3@...rix.com, mingo@...hat.com, bp@...en8.de
Subject: Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing
 high-probability event dispatching



On 1/16/25 21:54, Xin Li wrote:
> On 1/16/2025 9:18 PM, Ethan Zhao wrote:
>>>
>>> Just swap the 2 arguments, and it should be:
>>> +    switch_likely (etype, EVENT_TYPE_OTHER) {
>>>
>>>
>> after swapped the parameters as following:
>> +#define switch_likely(v,l) \
>> + switch((__typeof__(v))__builtin_expect((v),(l)))
>> +
>>   __visible noinstr void fred_entry_from_user(struct pt_regs *regs)
>>   {
>>          unsigned long error_code = regs->orig_ax;
>> +       unsigned short etype = regs->fred_ss.type & 0xf;
>>
>>          /* Invalidate orig_ax so that syscall_get_nr() works 
>> correctly */
>>          regs->orig_ax = -1;
>>
>> -       switch (regs->fred_ss.type) {
>> +       switch_likely (etype, (EVENT_TYPE_EXTINT == etype || 
>> EVENT_TYPE_OTHER == etype)) {
> 
> This is not what I suggested, the (l) argument should be only one
> constant; __builtin_expect() doesn't allow 2 different constants.
> 

The (l) argument is not a boolean expression! It is the *expected value* 
of (v).

	-hpa