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

[Ricardo Neri]

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@xxxxxxxxxxxxxxx> 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@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()?
> 
> 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@xxxxxxxxxxxxxxx/

Thanks and BR,
Ricardo