Messages in this thread | | | Date | Wed, 4 Mar 2015 08:13:24 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE_2 |
| |
* Ingo Molnar <mingo@kernel.org> wrote:
> > * 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.
Btw., the x86 memset() variants are using this today, and I think this is the most optimal jump-patching variant, even if it means a small amount of code duplication between the copy_user variants.
Thanks,
Ingo
| |