Messages in this thread | | | Date | Wed, 4 Mar 2015 07:25:52 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE_2 |
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* 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
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