linux-kernel - Re: [PATCH 3/4] x86/asm: delete dummy variables in movdir64b()

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <253E93DC-CC61-4AD5-AA87-81674B55C25A@zytor.com>
Date: Fri, 07 Mar 2025 03:54:35 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Ingo Molnar <mingo@...nel.org>, Alexey Dobriyan <adobriyan@...il.com>
CC: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/asm: delete dummy variables in movdir64b()

On March 7, 2025 3:49:26 AM PST, Ingo Molnar <mingo@...nel.org> wrote:
>
>* Alexey Dobriyan <adobriyan@...il.com> wrote:
>
>> Cast to pointer-to-array instead.
>> 
>> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
>> ---
>>  arch/x86/include/asm/special_insns.h | 9 +++------
>>  1 file changed, 3 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
>> index d349aa0f0a83..b24c6c945c38 100644
>> --- a/arch/x86/include/asm/special_insns.h
>> +++ b/arch/x86/include/asm/special_insns.h
>> @@ -215,13 +215,10 @@ static __always_inline void serialize(void)
>>  /* The dst parameter must be 64-bytes aligned */
>>  static inline void movdir64b(void *dst, const void *src)
>>  {
>> -	const struct { char _[64]; } *__src = src;
>> -	struct { char _[64]; } *__dst = dst;
>> -
>>  	/*
>>  	 * MOVDIR64B %(rdx), rax.
>>  	 *
>> -	 * Both __src and __dst must be memory constraints in order to tell the
>> +	 * Both src and dst must be memory constraints in order to tell the
>>  	 * compiler that no other memory accesses should be reordered around
>>  	 * this one.
>>  	 *
>> @@ -230,8 +227,8 @@ static inline void movdir64b(void *dst, const void *src)
>>  	 * I.e., not the pointers but what they point to, thus the deref'ing '*'.
>>  	 */
>>  	asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
>> -		     : "+m" (*__dst)
>> -		     :  "m" (*__src), "a" (__dst), "d" (__src));
>> +		     : "+m" (*(char(*)[64])dst)
>> +		     :  "m" (*(const char(*)[64])src), "a" (dst), "d" (src));
>
>In what world is putting type casts inside asm() statements an 
>improvement to the code?
>
>Thanks,
>
>	Ingo

I second that sentiment. This is a net negative.