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SubjectRe: [PATCH v2] hwmon: (dell-smm) Improve assembly code
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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@gmx.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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