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Message-ID: <alpine.DEB.2.02.1305071301040.4147@kaball.uk.xensource.com>
Date: Tue, 7 May 2013 13:15:02 +0100
From: Stefano Stabellini <stefano.stabellini@...citrix.com>
To: Ian Campbell <Ian.Campbell@...rix.com>
CC: Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
"linux@....linux.org.uk" <linux@....linux.org.uk>,
"arnd@...db.de" <arnd@...db.de>,
"marc.zyngier@....com" <marc.zyngier@....com>,
"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
"will.deacon@....com" <will.deacon@....com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"nico@...aro.org" <nico@...aro.org>,
"olof@...om.net" <olof@...om.net>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"cov@...eaurora.org" <cov@...eaurora.org>
Subject: Re: [Xen-devel] [PATCH RFC v2 2/4] arm: introduce CONFIG_PARAVIRT
and pv_time_ops
On Tue, 7 May 2013, Ian Campbell wrote:
> On Mon, 2013-05-06 at 15:51 +0100, Stefano Stabellini wrote:
> > Introduce CONFIG_PARAVIRT on ARM.
>
> What about PARAVIRT_TIME_ACCOUNTING? I'm not sure what it is but it
> looks like a more lightweight version of pv stolen time?
PARAVIRT_TIME_ACCOUNTING selects PARAVIRT on x86 :-)
> > The only paravirt interface supported is pv_time_ops.steal_clock.
> > No runtime pvops patching yet.
>
> Or indeed ever, I think. The use cases for patching on x86 are not
> things which carry over to ARM with virt extensions.
Agreed
> > This allows us to make us of steal_account_process_tick for stolen ticks
> > accounting.
> >
> >
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > CC: linux@....linux.org.uk
> > CC: will.deacon@....com
> > CC: nico@...aro.org
> > CC: marc.zyngier@....com
> > CC: cov@...eaurora.org
> > CC: arnd@...db.de
> > CC: olof@...om.net
> > ---
> > arch/arm/Kconfig | 9 +++++++++
> > arch/arm/include/asm/paravirt.h | 19 +++++++++++++++++++
> > arch/arm/kernel/Makefile | 1 +
> > arch/arm/kernel/paravirt.c | 32 ++++++++++++++++++++++++++++++++
> > 4 files changed, 61 insertions(+), 0 deletions(-)
> > create mode 100644 arch/arm/include/asm/paravirt.h
> > create mode 100644 arch/arm/kernel/paravirt.c
> >
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index 344e299..35cb10a 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -1887,12 +1887,21 @@ config XEN_DOM0
> > def_bool y
> > depends on XEN
> >
> > +config PARAVIRT
> > + bool "Enable paravirtualization code"
> > + ---help---
> > + This changes the kernel so it can modify itself when it is run
> > + under a hypervisor, potentially improving performance significantly
> > + over full virtualization. However, when run without a hypervisor
> > + the kernel is theoretically slower and slightly larger.
>
> I'm not sure this description (carried over from x86) are really true
> for ARM. e.g. the downsides there when not virtualised are in the PV MMU
> (pte operations) and interrupt masking stuff, which should never make
> its way onto ARM.
Right
> I think it would be a worthwhile change to refactor the stolen time
> handling out from under the rather wide reaching umbrella of the x86
> PARAVIRT option. (assuming PARAVIRT_TIME_ACCOUNTING isn't already that)
Actually PARAVIRT doesn't mean much in common code, the only thing it
covers is stolen time.
What I mean to say is that just because we are introducing something
called "PARAVIRT" on ARM, it doesn't mean that it has to come with all
sort of baggage.
> > +
> > config XEN
> > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > depends on ARM && AEABI && OF
> > depends on CPU_V7 && !CPU_V6
> > depends on !GENERIC_ATOMIC64
> > select ARM_PSCI
> > + select PARAVIRT
> > help
> > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> >
> > diff --git a/arch/arm/include/asm/paravirt.h b/arch/arm/include/asm/paravirt.h
> > new file mode 100644
> > index 0000000..3b95bc6
> > --- /dev/null
> > +++ b/arch/arm/include/asm/paravirt.h
> > @@ -0,0 +1,19 @@
> > +#ifndef _ASM_ARM_PARAVIRT_H
> > +#define _ASM_ARM_PARAVIRT_H
> > +
> > +struct static_key;
> > +extern struct static_key paravirt_steal_enabled;
> > +extern struct static_key paravirt_steal_rq_enabled;
> > +
> > +struct pv_time_ops {
> > + unsigned long long (*steal_clock)(int cpu);
> > +};
> > +extern struct pv_time_ops pv_time_ops;
> > +
> > +static inline u64 paravirt_steal_clock(int cpu)
> > +{
> > + return pv_time_ops.steal_clock(cpu);
> > +}
> > +
> > +
> > +#endif
> > diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
> > index dd9d90a..6764f60 100644
> > --- a/arch/arm/kernel/Makefile
> > +++ b/arch/arm/kernel/Makefile
> > @@ -86,5 +86,6 @@ ifeq ($(CONFIG_ARM_PSCI),y)
> > obj-y += psci.o
> > obj-$(CONFIG_SMP) += psci_smp.o
> > endif
> > +obj-$(CONFIG_PARAVIRT) += paravirt.o
> >
> > extra-y := $(head-y) vmlinux.lds
> > diff --git a/arch/arm/kernel/paravirt.c b/arch/arm/kernel/paravirt.c
> > new file mode 100644
> > index 0000000..3e73fc8
> > --- /dev/null
> > +++ b/arch/arm/kernel/paravirt.c
> > @@ -0,0 +1,32 @@
> > +/*
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > + * GNU General Public License for more details.
> > + *
> > + * Copyright (C) 2013 Citrix Systems
> > + *
> > + * Author: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > + */
> > +
> > +#include <linux/export.h>
> > +#include <linux/jump_label.h>
> > +#include <linux/types.h>
> > +#include <asm/paravirt.h>
> > +
> > +struct static_key paravirt_steal_enabled;
> > +struct static_key paravirt_steal_rq_enabled;
> > +
> > +static u64 native_steal_clock(int cpu)
> > +{
> > + return 0;
> > +}
> > +
> > +struct pv_time_ops pv_time_ops = {
> > + .steal_clock = native_steal_clock,
> > +};
> > +EXPORT_SYMBOL_GPL(pv_time_ops);
>
> This foo_ops.bar and native_bar thing is a bit of a hangover from the
> paravirt patching infrastructure on x86 and it doesn't really apply
> here.
>
> Given that the call to paravirt_steal_time call is already protected by
> this static_key stuff I think it would be safe to leave the hook as NULL
> in the case where it is unused.
Good point
> Given all the different clock sources on ARM is there not an existing
> ops struct where this could live?
I am not sure, but I would be happy to move it something arch-specific.
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