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    SubjectRe: [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure
    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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