Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

On Thu, Oct 05, 2017 at 04:35:03PM -0400, Boris Ostrovsky wrote:
> 
> >  #ifdef CONFIG_PARAVIRT
> > +/*
> > + * Paravirt alternatives are applied much earlier than normal alternatives.
> > + * They are only applied when running on a hypervisor.  They replace some
> > + * native instructions with calls to pv ops.
> > + */
> > +void __init apply_pv_alternatives(void)
> > +{
> > +   setup_force_cpu_cap(X86_FEATURE_PV_OPS);
> 
> Not for Xen HVM guests.

From what I can tell, HVM guests still use pv_time_ops and
pv_mmu_ops.exit_mmap, right?

> > +   apply_alternatives(__pv_alt_instructions, __pv_alt_instructions_end);
> > +}
> 
> 
> This is a problem (at least for Xen PV guests):
> apply_alternatives()->text_poke_early()->local_irq_save()->...'cli'->death.

Ah, right.

> It might be possible not to turn off/on the interrupts in this
> particular case since the guest probably won't be able to handle an
> interrupt at this point anyway.

Yeah, that should work.  For Xen and for the other hypervisors, this is
called well before irq init, so interrupts can't be handled yet anyway.

> > +
> >  void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
> >                                  struct paravirt_patch_site *end)
> >  {
> > diff --git a/arch/x86/kernel/cpu/hypervisor.c 
> > b/arch/x86/kernel/cpu/hypervisor.c
> > index 4fa90006ac68..17243fe0f5ce 100644
> > --- a/arch/x86/kernel/cpu/hypervisor.c
> > +++ b/arch/x86/kernel/cpu/hypervisor.c
> > @@ -71,6 +71,8 @@ void __init init_hypervisor_platform(void)
> >     if (!x86_hyper)
> >             return;
> >  
> > +   apply_pv_alternatives();
> 
> Not for Xen PV guests who have already done this.

I think it would be harmless, but yeah, it's probably best to only write
it once.

Thanks for the review!

-- 
Josh

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