From mboxrd@z Thu Jan  1 00:00:00 1970
From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH 11/13] x86/paravirt: Add paravirt alternatives
	infrastructure
Date: Fri, 6 Oct 2017 09:32:59 -0500
Message-ID: <20171006143259.rs3zh7k5tmsgesqy__32890.1692796082$1509080056$gmane$org@treble>
References: <cover.1507128293.git.jpoimboe@redhat.com>
	<39743c79546ede3073586403d0836a4f93519b0a.1507128293.git.jpoimboe@redhat.com>
	<3b9fd404-6912-3b58-db29-36202631b438@oracle.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <3b9fd404-6912-3b58-db29-36202631b438@oracle.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>, Rusty Russell <rusty@rustcorp.com.au>, Mike Galbraith <efault@gmx.de>, xen-devel@lists.xenproject.org, Peter Zijlstra <peterz@infradead.org>, Jiri Slaby <jslaby@suse.cz>, x86@kernel.org, linux-kernel@vger.kernel.org, Sasha Levin <alexander.levin@verizon.com>, Chris Wright <chrisw@sous-sol.org>, Thomas Gleixner <tglx@linutronix.de>, Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>, live-patching@vger.kernel.org, Alok Kataria <akataria@vmware.com>, virtualization@lists.linux-foundation.org, Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@kernel.org>
List-Id: virtualization@lists.linuxfoundation.org

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