On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote: > On Tue, Aug 04, 2020 at 09:58:25PM -0700, h...@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()? Thanks and BR, Ricardo