linux-kernel - Re: [PATCH v2] hwmon: (dell-smm) Improve assembly code

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Date:   Sun, 20 Feb 2022 07:30:01 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     David Laight <David.Laight@...LAB.COM>,
        'Armin Wolf' <W_Armin@....de>,
        "pali@...nel.org" <pali@...nel.org>
Cc:     "jdelvare@...e.com" <jdelvare@...e.com>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-assembly@...r.kernel.org" <linux-assembly@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hwmon: (dell-smm) Improve assembly code

On 2/20/22 04:20, David Laight wrote:
> From: Armin Wolf
>> Sent: 19 February 2022 21:10
>>
>> The new assembly code works on both 32 bit and 64 bit
>> cpus and allows for more compiler optimisations by not
>> requiring smm_regs to be packed. Also since the
>> SMM handler seems to modify the carry flag, the new
>> code informs the compiler that the flags register
>> needs to be saved/restored. Since clang runs out of
>> registers on 32 bit x86 when using CC_OUT, we need
>> to execute "setc" ourself.
> 
> You always need to save anything from the flags register
> inside the asm block - it is never valit afterwards.
> 

Does that matter here ? I thought setcc is used to get the carry flag.

Guenter

>> Also modify the debug message so we can still see
>> the result (eax) when the carry flag was set.
>>
>> Tested with 32 bit and 64 bit kernels on a Dell Inspiron 3505.
>>
>> Signed-off-by: Armin Wolf <W_Armin@....de>
>> ---
>> Changes in v2:
>> - fix clang running out of registers on 32 bit x86
>> - modify debug message
>> ---
>>   drivers/hwmon/dell-smm-hwmon.c | 85 ++++++++++------------------------
>>   1 file changed, 25 insertions(+), 60 deletions(-)
>>
>> diff --git a/drivers/hwmon/dell-smm-hwmon.c b/drivers/hwmon/dell-smm-hwmon.c
>> index c5939e68586d..f1538a46bfc9 100644
>> --- a/drivers/hwmon/dell-smm-hwmon.c
>> +++ b/drivers/hwmon/dell-smm-hwmon.c
>> @@ -119,7 +119,7 @@ struct smm_regs {
>>   	unsigned int edx;
>>   	unsigned int esi;
>>   	unsigned int edi;
>> -} __packed;
>> +};
>>
>>   static const char * const temp_labels[] = {
>>   	"CPU",
>> @@ -165,73 +165,38 @@ static int i8k_smm_func(void *par)
>>   	int eax = regs->eax;
>>   	int ebx = regs->ebx;
>>   	long long duration;
>> -	int rc;
>> +	bool carry;
> 
> I'd use an explicit 'unsigned char' not bool.
> Matches the type of the 'setcc' instriction.
> 
>>   	/* SMM requires CPU 0 */
>>   	if (smp_processor_id() != 0)
>>   		return -EBUSY;
>>
>> -#if defined(CONFIG_X86_64)
>> -	asm volatile("pushq %%rax\n\t"
>> -		"movl 0(%%rax),%%edx\n\t"
>> -		"pushq %%rdx\n\t"
>> -		"movl 4(%%rax),%%ebx\n\t"
>> -		"movl 8(%%rax),%%ecx\n\t"
>> -		"movl 12(%%rax),%%edx\n\t"
>> -		"movl 16(%%rax),%%esi\n\t"
>> -		"movl 20(%%rax),%%edi\n\t"
>> -		"popq %%rax\n\t"
>> -		"out %%al,$0xb2\n\t"
>> -		"out %%al,$0x84\n\t"
>> -		"xchgq %%rax,(%%rsp)\n\t"
>> -		"movl %%ebx,4(%%rax)\n\t"
>> -		"movl %%ecx,8(%%rax)\n\t"
>> -		"movl %%edx,12(%%rax)\n\t"
>> -		"movl %%esi,16(%%rax)\n\t"
>> -		"movl %%edi,20(%%rax)\n\t"
>> -		"popq %%rdx\n\t"
>> -		"movl %%edx,0(%%rax)\n\t"
>> -		"pushfq\n\t"
>> -		"popq %%rax\n\t"
>> -		"andl $1,%%eax\n"
>> -		: "=a"(rc)
>> -		:    "a"(regs)
>> -		:    "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");
>> -#else
>> -	asm volatile("pushl %%eax\n\t"
>> -	    "movl 0(%%eax),%%edx\n\t"
>> -	    "push %%edx\n\t"
>> -	    "movl 4(%%eax),%%ebx\n\t"
>> -	    "movl 8(%%eax),%%ecx\n\t"
>> -	    "movl 12(%%eax),%%edx\n\t"
>> -	    "movl 16(%%eax),%%esi\n\t"
>> -	    "movl 20(%%eax),%%edi\n\t"
>> -	    "popl %%eax\n\t"
>> -	    "out %%al,$0xb2\n\t"
>> -	    "out %%al,$0x84\n\t"
>> -	    "xchgl %%eax,(%%esp)\n\t"
>> -	    "movl %%ebx,4(%%eax)\n\t"
>> -	    "movl %%ecx,8(%%eax)\n\t"
>> -	    "movl %%edx,12(%%eax)\n\t"
>> -	    "movl %%esi,16(%%eax)\n\t"
>> -	    "movl %%edi,20(%%eax)\n\t"
>> -	    "popl %%edx\n\t"
>> -	    "movl %%edx,0(%%eax)\n\t"
>> -	    "lahf\n\t"
>> -	    "shrl $8,%%eax\n\t"
>> -	    "andl $1,%%eax\n"
>> -	    : "=a"(rc)
>> -	    :    "a"(regs)
>> -	    :    "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");
>> -#endif
>> -	if (rc != 0 || (regs->eax & 0xffff) == 0xffff || regs->eax == eax)
>> -		rc = -EINVAL;
>> +	asm volatile("out %%al,$0xb2\n\t"
>> +		     "out %%al,$0x84\n\t"
>> +		     "setc %0\n"
>> +		     : "=mr" (carry),
>> +		       "=a" (regs->eax),
>> +		       "=b" (regs->ebx),
>> +		       "=c" (regs->ecx),
>> +		       "=d" (regs->edx),
>> +		       "=S" (regs->esi),
>> +		       "=D" (regs->edi)
>> +		     : "a" (regs->eax),
>> +		       "b" (regs->ebx),
>> +		       "c" (regs->ecx),
>> +		       "d" (regs->edx),
>> +		       "S" (regs->esi),
>> +		       "D" (regs->edi)
> 
> If you use "+a" (etc) for the output registers you don't
> need to respecify them as input registers.
> 
>> +		     : "cc");
> 
> No need to specify "cc", it is always assumed clobbered.
> 
> 	David
> 
>>
>>   	duration = ktime_us_delta(ktime_get(), calltime);
>> -	pr_debug("smm(0x%.4x 0x%.4x) = 0x%.4x  (took %7lld usecs)\n", eax, ebx,
>> -		 (rc ? 0xffff : regs->eax & 0xffff), duration);
>> +	pr_debug("smm(0x%.4x 0x%.4x) = 0x%.4x carry: %d (took %7lld usecs)\n",
>> +		 eax, ebx, regs->eax & 0xffff, carry, duration);
>>
>> -	return rc;
>> +	if (carry || (regs->eax & 0xffff) == 0xffff || regs->eax == eax)
>> +		return -EINVAL;
>> +
>> +	return 0;
>>   }
>>
>>   /*
>> --
>> 2.30.2
> 
> -
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