On 07/31/2014 02:41 AM, kan.li...@intel.com wrote:
> From: Kan Liang <kan.li...@intel.com>
>
> Currently, {rd,wr}msrl_safe can handle the exception which caused by accessing
> specific MSR.
> However, it will introduce extra conditional branch for testing errors. That
> will impact the "fast" path's performance.
> The newly implemented {rd,wr}msrl_goto function can not only handle the
> exception which caused by accessing specific MSR,
> but also takes advantage of the asm goto extension to eliminate the impact of
> performance.
>
> The asm goto extension is supported by GCC 4.5 and later versions. If the
> compiler doesn't support goto extension, _safe will be used to replace _goto.
>
> Signed-off-by: Kan Liang <kan.li...@intel.com>
> ---
> arch/x86/include/asm/msr.h | 60
> +++++++++++++++++++++++++++++++++++++++++
> arch/x86/include/asm/paravirt.h | 18 +++++++++++++
> 2 files changed, 78 insertions(+)
>
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index de36f22..55438da 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -203,6 +203,66 @@ do {
> \
>
> #define rdtscpll(val, aux) (val) = native_read_tscp(&(aux))
>
> +#ifdef CC_HAVE_ASM_GOTO
> +
> +/*
> + * The _goto version is rdmsrl/wrmsrl with exception handling
> + * The advantage (than _safe) is that it can directly jump in the
> + * exception handling code, and never test in the "fast" path.
> + *
> + * Since _goto doesn't support output, try to protect the output
> + * registers by clobbers, and process the registers immediately.
> + */
> +#define rdmsrl_goto(msr, result, fail_label) \
> +do { \
> + DECLARE_ARGS(val, low, high); \
> + asm_volatile_goto("2: rdmsr\n" \
> + "1:\n\t" \
> + _ASM_EXTABLE(2b, %l[fail_label]) \
> + : /* No outputs. */ \
> + : "c" (msr) \
> + : "%rax", "%rdx" \
> + : fail_label); \
> + asm volatile ("" \
> + : EAX_EDX_RET(val, low, high) \
> + : ); \
This is scary -- the compiler is free to optimize this incorrectly, and
it doesn't even seem very farfetched to me.
> + result = EAX_EDX_VAL(val, low, high); \
> +} while (0)
> +
> +#define wrmsrl_goto(msr, val, fail_label) \
> +do { \
> + unsigned low, high; \
> + low = (u32)val; \
> + high = (u32)(val >> 32); \
> + asm_volatile_goto("2: wrmsr\n" \
> + "1:\n\t" \
> + _ASM_EXTABLE(2b, %l[fail_label]) \
> + : /* No outputs. */ \
> + : "c" (msr), "a" (low), "d" (high) \
> + : "memory" \
> + : fail_label); \
> +} while (0)
I like this one.
--Andy
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