Re: [Xen-devel] [PATCH RFC v2 2/4] arm: introduce CONFIG_PARAVIRTand pv_time_ops

[Ian Campbell]

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?

> 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.

> This allows us to make us of steal_account_process_tick for stolen ticks
> accounting.
> 
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> CC: linux@xxxxxxxxxxxxxxxx
> CC: will.deacon@xxxxxxx
> CC: nico@xxxxxxxxxx
> CC: marc.zyngier@xxxxxxx
> CC: cov@xxxxxxxxxxxxxx
> CC: arnd@xxxxxxxx
> CC: olof@xxxxxxxxx
> ---
>  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.

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)

> +
>  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@xxxxxxxxxxxxx>
> + */
> +
> +#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.

Given all the different clock sources on ARM is there not an existing
ops struct where this could live?

Ian.

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