linux-kernel - Re: [PATCH v2] x86/vdso: Use RDPID in preference to LSL when available

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Message-ID: <20160421121640.GC28821@pd.tnic>
Date:	Thu, 21 Apr 2016 14:16:40 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/vdso: Use RDPID in preference to LSL when
 available

On Wed, Apr 20, 2016 at 06:16:01PM -0700, Andy Lutomirski wrote:
> Also, it's time for someone to do UMIP.  I'll see if I can convince
> someone in KVM land to emulate it to make it easier to test.

That'll be fun - we can simply set that bit in CR4 and see who screams
:-P

> Changes from v1:
>  - Remove rdpid() from special_instructions.h.  (Was a leftover -- sorry.)
> 
> arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/include/asm/vgtod.h       | 7 ++++++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 7bfb6b70c745..beaf2fb601ee 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -279,6 +279,7 @@
>  /* Intel-defined CPU features, CPUID level 0x00000007:0 (ecx), word 16 */
>  #define X86_FEATURE_PKU		(16*32+ 3) /* Protection Keys for Userspace */
>  #define X86_FEATURE_OSPKE	(16*32+ 4) /* OS Protection Keys Enable */
> +#define X86_FEATURE_RDPID	(16*32+ 22) /* RDPID instruction */
>  
>  /*
>   * BUG word(s)
> diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
> index e728699db774..3a01996db58f 100644
> --- a/arch/x86/include/asm/vgtod.h
> +++ b/arch/x86/include/asm/vgtod.h
> @@ -89,8 +89,13 @@ static inline unsigned int __getcpu(void)
>  	 * works on all CPUs.  This is volatile so that it orders
>  	 * correctly wrt barrier() and to keep gcc from cleverly
>  	 * hoisting it out of the calling function.
> +	 *
> +	 * If RDPID is available, use it.
>  	 */
> -	asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
> +	alternative_io ("lsl %[p],%[seg]",
> +			".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */

AFAICT, 0xf8 is correct, if I'm reading the SDM right:

bits [7:6] must be 11b for opcode group 9 and RDPID is in the 11b row,
bits [5:3] are ModRM.reg and they need to be 111b for RDPID (0x7 column)
and the last three [2:0] select the register and they must be 000b for
rAX.

HOWEVER, you need to make the asm output register constraint "=a"
because you're specifying rAX as a destination register for RDPID.

Also, I'm wondering: should we supply that alternative in a separate
inline function in special_instructions.h for wider use? I.e., something
like read_cpu_num() or so...

-- 
Regards/Gruss,
    Boris.

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