Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE_2

[Ingo Molnar <mingo@kernel.org>]

* Borislav Petkov <bp@alien8.de> wrote:

> From: Borislav Petkov <bp@suse.de>
> 
> Use the asm macro and drop the locally grown version.

> @@ -73,9 +49,11 @@ ENTRY(_copy_to_user)
>  	jc bad_to_user
>  	cmpq TI_addr_limit(%rax),%rcx
>  	ja bad_to_user
> +	ALTERNATIVE_2 "jmp copy_user_generic_unrolled",		\
> +		      "jmp copy_user_generic_string",		\
> +		      X86_FEATURE_REP_GOOD,			\
> +		      "jmp copy_user_enhanced_fast_string",	\
> +		      X86_FEATURE_ERMS

Btw., as a future optimization, wouldn't it be useful to patch this 
function at its first instruction, i.e. to have three fully functional 
copy_user_generic_ variants and choose to jmp to one of them in the 
first instruction of the original function?

The advantage would be two-fold:

 1) right now: smart microarchitectures that are able to optimize
    jump-after-jump (and jump-after-call) targets in their branch
    target cache can do so in this case, reducing the overhead of the
    patching, possibly close to zero in the cached case.

 2) in the future: we could actually do a (limited) re-link of the
    kernel during bootup, and patch up the original copy_to_user call
    sites directly to one of the three variants. Alternatives patching
    done at the symbol level. Does current tooling allow something
    like this already?

Thanks,

	Ingo