On Thu, Jun 18, 2015 at 03:59:06AM -0400, George Spelvin wrote:
> Before, the code to do rdtsc looked like:
> rdtsc
> shl $0x20,%rdx
> mov %eax,%eax
> or %rdx,%rax
>
> The "mov %eax,%eax" is required to clear the high 32 bits of %rax.
>
> By declaring low and high as 64-bit variables, the code is simplified to
> rdtsc
> shl $0x20,%rdx
> or %rdx,%rax
>
> Yes, it's a 2-byte instruction that's not on a critical path, but
> there are principles to be upheld.
>
> Every user of EAX_EDX_RET has been checked. I tried to check users of
> EAX_EDX_ARGS, but there weren't any, so I deleted it to be safe.
>
> (There's no benefit to making "high" 64 bits, but it was the simplest
> way to proceed.)
>
> Signed-off-by: George Spelvin <linux@xxxxxxxxxxx>
> ---
> I came across this while in the middle of hacking on some other code that
> isn't booting yet, so I haven't exactly tested it thoroughly. I *did*
> check both the Intel and AMD specifications to ensure that this was a safe
> optimization, and did user-space tests on both Intel and AMD processors.
>
> But it's simple enough I'm willing to risk public embarrassment.
>
> arch/x86/include/asm/msr.h | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index de36f22e..d6b8a80d 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -46,14 +46,13 @@ static inline unsigned long long native_read_tscp(unsigned int *aux)
> * it means rax *or* rdx.
> */
> #ifdef CONFIG_X86_64
> -#define DECLARE_ARGS(val, low, high) unsigned low, high
> -#define EAX_EDX_VAL(val, low, high) ((low) | ((u64)(high) << 32))
> -#define EAX_EDX_ARGS(val, low, high) "a" (low), "d" (high)
> +/* 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) unsigned long long val
> #define EAX_EDX_VAL(val, low, high) (val)
> -#define EAX_EDX_ARGS(val, low, high) "A" (val)
> #define EAX_EDX_RET(val, low, high) "=A" (val)
> #endif
Looks ok to me, I'll pick it up along with Andy's TSC cleanup. CCed.
Thanks.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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