linux-kernel - Re: [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

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Date:   Thu, 5 Oct 2017 16:35:03 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        live-patching@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>,
        Chris Wright <chrisw@...s-sol.org>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        virtualization@...ts.linux-foundation.org,
        xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 11/13] x86/paravirt: Add paravirt alternatives
 infrastructure


>  #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.

> +	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.

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.


> +
>  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.

-boris


> +
>  	if (x86_hyper->init_platform)
>  		x86_hyper->init_platform();
>  }
>