Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync_core() when available

[Ricardo Neri]

On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote:
> On Tue, Aug 04, 2020 at 09:58:25PM -0700, hpa@xxxxxxxxx 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