Re: [PATCH v8 3/3] x86, mce: Add __mcsafe_copy()

[Borislav Petkov <bp@alien8.de>]

On Fri, Jan 29, 2016 at 04:35:35PM -0800, Tony Luck wrote:
> On Wed, Jan 13, 2016 at 8:39 PM, Borislav Petkov <bp@alien8.de> wrote:
> > On Wed, Jan 13, 2016 at 03:22:58PM -0800, Tony Luck wrote:
> >> Are there some examples of synthetic CPUID bits?
> >
> > X86_FEATURE_ALWAYS is one. The others got renamed into X86_BUG_* ones,
> > the remaining mechanism is the same, though.
> 
> So something like this [gmail will line wrap, but should still be legible]
> 
> Then Dan will be able to use:
> 
>       if (cpu_has(c, X86_FEATURE_MCRECOVERY))
> 
> to decide whether to use the (slightly slower, but recovery capable)
> __mcsafe_copy()
> or just pick the fastest memcpy() instead.

The most optimal way of alternatively calling two functions would be
something like this, IMO:

alternative_call(memcpy, __mcsafe_copy, X86_FEATURE_MCRECOVERY,
		 ASM_OUTPUT2("=a" (mcsafe_ret.trapnr), "=d" (mcsafe_ret.remain)),
		 "D" (dst), "S" (src), "d" (len));

I hope I've not messed up the calling convention but you want the inputs
in %rdi, %rsi, %rdx and the outputs in %rax, %rdx, respectively. Just
check the asm gcc generates and do not trust me :)

The other thing you probably would need to do is create our own
__memcpy() which returns struct mcsafe_ret so that the signatures of
both functions match.

Yeah, it is a bit of jumping through hoops but this way we do a CALL
<func_ptr> directly in asm, without any JMPs or NOPs padding the other
alternatives methods add.

But if you don't care about a small JMP and that is not a hot path, you
could do the simpler:

	if (static_cpu_has(X86_FEATURE_MCRECOVERY))
		return __mcsafe_copy(...);

	return memcpy();

which adds a JMP or a 5-byte NOP depending on the X86_FEATURE_MCRECOVERY
setting.

> diff --git a/arch/x86/include/asm/cpufeature.h
> b/arch/x86/include/asm/cpufeature.h
> index 7ad8c9464297..621e05103633 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -106,6 +106,7 @@
>  #define X86_FEATURE_APERFMPERF ( 3*32+28) /* APERFMPERF */
>  #define X86_FEATURE_EAGER_FPU  ( 3*32+29) /* "eagerfpu" Non lazy FPU restore */
>  #define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in
> S3 state */
> +#define X86_FEATURE_MCRECOVERY ( 3*32+31) /* cpu has recoverable

Why not write it out?

	X86_FEATURE_MCE_RECOVERY

> machine checks */
> 
>  /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
>  #define X86_FEATURE_XMM3       ( 4*32+ 0) /* "pni" SSE-3 */

-- 
Regards/Gruss,
    Boris.

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