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

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Message-ID:
 <TYZPR03MB88015F2A7F8BE1C32B57C849D11B2@TYZPR03MB8801.apcprd03.prod.outlook.com>
Date: Fri, 17 Jan 2025 14:58:25 +0800
From: Ethan Zhao <etzhao@...look.com>
To: Xin Li <xin@...or.com>, Ethan Zhao <haifeng.zhao@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.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 1/17/2025 1:54 PM, 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.

That is why it couldn't be used in switch(var), while it works with if(true) else if (false).
Switch(var) needs a var returned by the __builtin_expect(), but it only could return Const.

Thanks,
Ethan