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

[Guenter Roeck <linux@xxxxxxxxxxxx>]

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@xxxxxx>
> > ---
> > 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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