Messages in this thread | | | Date | Wed, 5 Aug 2020 12:11:26 -0700 | From | Ricardo Neri <> | Subject | Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync_core() when available |
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On Wed, Aug 05, 2020 at 11:28:31AM -0700, Andy Lutomirski wrote: > On Wed, Aug 5, 2020 at 10:07 AM Ricardo Neri > <ricardo.neri-calderon@linux.intel.com> wrote: > > > > On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote: > > > On Tue, Aug 04, 2020 at 09:58:25PM -0700, hpa@zytor.com wrote: > > > > Because why use an alternative to jump over one instruction? > > > > > > > > I personally would prefer to have the IRET put out of line > > > > > > Can't yet - SERIALIZE CPUs are a minority at the moment. > > > > > > > and have the call/jmp replaced by SERIALIZE inline. > > > > > > Well, we could do: > > > > > > alternative_io("... IRET bunch", __ASM_SERIALIZE, X86_FEATURE_SERIALIZE, ...); > > > > > > and avoid all kinds of jumping. Alternatives get padded so there > > > would be a couple of NOPs following when SERIALIZE gets patched in > > > but it shouldn't be a problem. I guess one needs to look at what gcc > > > generates... > > > > But the IRET-TO-SELF code has instruction which modify the stack. This > > would violate stack invariance in alternatives as enforced in commit > > 7117f16bf460 ("objtool: Fix ORC vs alternatives"). As a result, objtool > > gives warnings as follows: > > > > arch/x86/kernel/alternative.o: warning: objtool: do_sync_core()+0xe: > > alternative modifies stack > > > > Perhaps in this specific case it does not matter as the changes in the > > stack will be undone by IRET. However, using alternative_io would require > > adding the macro STACK_FRAME_NON_STANDARD to functions using sync_core(). > > IMHO, it wouldn't look good. > > > > So maybe the best approach is to implement as you suggested using > > static_cpu_has()? > > I agree. Let's keep it simple. > > Honestly, I think the right solution is to have iret_to_self() in > actual asm and invoke it from C as needed.
Do you mean anything different from what we have already [1]? If I understand your comment correctly, we have exactly that: an iret_to_self() asm implementation invoked from C.
Thanks and BR, Ricardo
[1]. https://lore.kernel.org/lkml/20200727043132.15082-4-ricardo.neri-calderon@linux.intel.com/
Thanks and BR, Ricardo
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