linux-kernel - Re: [PATCH v4 02/15] x86/msr: Move rdtsc{,

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Message-ID: <2cbb468c-188e-4e6b-9b17-b60a66208c7a@zytor.com>
Date: Fri, 2 May 2025 11:09:29 -0700
From: Xin Li <xin@...or.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
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Subject: Re: [PATCH v4 02/15] x86/msr: Move rdtsc{,_ordered}() to <asm/tsc.h>

On 5/2/2025 1:02 AM, Ingo Molnar wrote:
> 
> * Xin Li (Intel) <xin@...or.com> wrote:
> 
>> index 94408a784c8e..13335a130edf 100644
>> --- a/arch/x86/include/asm/tsc.h
>> +++ b/arch/x86/include/asm/tsc.h
>> @@ -7,7 +7,81 @@
>>   
>>   #include <asm/cpufeature.h>
>>   #include <asm/processor.h>
>> -#include <asm/msr.h>
>> +
>> +/*
>> + * both i386 and x86_64 returns 64-bit value in edx:eax, but gcc's "A"
>> + * constraint has different meanings. For i386, "A" means exactly
>> + * edx:eax, while for x86_64 it doesn't mean rdx:rax or edx:eax. Instead,
>> + * it means rax *or* rdx.
>> + */
>> +#ifdef CONFIG_X86_64
>> +/* Using 64-bit values saves one instruction clearing the high half of low */
>> +#define DECLARE_ARGS(val, low, high)	unsigned long low, high
>> +#define EAX_EDX_VAL(val, low, high)	((low) | (high) << 32)
>> +#define EAX_EDX_RET(val, low, high)	"=a" (low), "=d" (high)
>> +#else
>> +#define DECLARE_ARGS(val, low, high)	u64 val
>> +#define EAX_EDX_VAL(val, low, high)	(val)
>> +#define EAX_EDX_RET(val, low, high)	"=A" (val)
>> +#endif
> 
> Meh, this patch creates a duplicate copy of DECLARE_ARGS() et al in
> <asm/tsc.h> now:
> 
>   arch/x86/include/asm/msr.h:#define DECLARE_ARGS(val, low, high) unsigned long low, high
>   arch/x86/include/asm/msr.h:#define DECLARE_ARGS(val, low, high) u64 val
>   arch/x86/include/asm/msr.h:     DECLARE_ARGS(val, low, high);
>   arch/x86/include/asm/msr.h:     DECLARE_ARGS(val, low, high);
>   arch/x86/include/asm/msr.h:     DECLARE_ARGS(val, low, high);
>   arch/x86/include/asm/tsc.h:#define DECLARE_ARGS(val, low, high) unsigned long low, high
>   arch/x86/include/asm/tsc.h:#define DECLARE_ARGS(val, low, high) u64 val
>   arch/x86/include/asm/tsc.h:     DECLARE_ARGS(val, low, high);
>   arch/x86/include/asm/tsc.h:     DECLARE_ARGS(val, low, high);
>   arch/x86/include/asm/tsc.h:#undef DECLARE_ARGS
> 
> Which was both an undeclared change, bloats the code, causes various
> problems, and is totally unnecessary to boot.
> 
> Please don't do that ...

Learned!

Especially that every change needs to explicitly called out.