Re: [patch v2 3/3] cpuidle-haltpoll: disable host side polling when kvm virtualized

[Joao Martins <joao.m.martins@oracle.com>]

On 6/4/19 1:24 PM, Marcelo Tosatti wrote:
> 
> When performing guest side polling, it is not necessary to 
> also perform host side polling. 
> 
> So disable host side polling, via the new MSR interface, 
> when loading cpuidle-haltpoll driver.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> ---
> 
> v2: remove extra "}"
> 
>  arch/x86/Kconfig                        |    7 +++++
>  arch/x86/include/asm/cpuidle_haltpoll.h |    8 ++++++
>  arch/x86/kernel/kvm.c                   |   40 ++++++++++++++++++++++++++++++++
>  drivers/cpuidle/cpuidle-haltpoll.c      |    9 ++++++-
>  include/linux/cpuidle_haltpoll.h        |   16 ++++++++++++
>  5 files changed, 79 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6.git/arch/x86/include/asm/cpuidle_haltpoll.h
> ===================================================================
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6.git/arch/x86/include/asm/cpuidle_haltpoll.h	2019-06-03 19:38:42.328718617 -0300
> @@ -0,0 +1,8 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ARCH_HALTPOLL_H
> +#define _ARCH_HALTPOLL_H
> +
> +void arch_haltpoll_enable(void);
> +void arch_haltpoll_disable(void);
> +
> +#endif
> Index: linux-2.6.git/drivers/cpuidle/cpuidle-haltpoll.c
> ===================================================================
> --- linux-2.6.git.orig/drivers/cpuidle/cpuidle-haltpoll.c	2019-06-03 19:38:12.376619124 -0300
> +++ linux-2.6.git/drivers/cpuidle/cpuidle-haltpoll.c	2019-06-03 19:38:42.328718617 -0300
> @@ -15,6 +15,7 @@
>  #include <linux/module.h>
>  #include <linux/timekeeping.h>
>  #include <linux/sched/idle.h>
> +#include <linux/cpuidle_haltpoll.h>
>  #define CREATE_TRACE_POINTS
>  #include "cpuidle-haltpoll-trace.h"
>  
> @@ -157,11 +158,17 @@
>  
>  static int __init haltpoll_init(void)
>  {
> -	return cpuidle_register(&haltpoll_driver, NULL);
> +	int ret = cpuidle_register(&haltpoll_driver, NULL);
> +
> +	if (ret == 0)
> +		arch_haltpoll_enable();
> +
> +	return ret;
>  }
>  
>  static void __exit haltpoll_exit(void)
>  {
> +	arch_haltpoll_disable();
>  	cpuidle_unregister(&haltpoll_driver);
>  }
>  
> Index: linux-2.6.git/include/linux/cpuidle_haltpoll.h
> ===================================================================
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6.git/include/linux/cpuidle_haltpoll.h	2019-06-03 19:41:57.293366260 -0300
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _CPUIDLE_HALTPOLL_H
> +#define _CPUIDLE_HALTPOLL_H
> +
> +#ifdef CONFIG_ARCH_CPUIDLE_HALTPOLL
> +#include <asm/cpuidle_haltpoll.h>
> +#else
> +static inline void arch_haltpoll_enable(void)
> +{
> +}
> +
> +static inline void arch_haltpoll_disable(void)
> +{
> +}
> +#endif
> +#endif
> Index: linux-2.6.git/arch/x86/Kconfig
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/Kconfig	2019-06-03 19:38:12.376619124 -0300
> +++ linux-2.6.git/arch/x86/Kconfig	2019-06-03 19:42:34.478489868 -0300
> @@ -787,6 +787,7 @@
>  	bool "KVM Guest support (including kvmclock)"
>  	depends on PARAVIRT
>  	select PARAVIRT_CLOCK
> +	select ARCH_CPUIDLE_HALTPOLL
>  	default y
>  	---help---
>  	  This option enables various optimizations for running under the KVM
> @@ -795,6 +796,12 @@
>  	  underlying device model, the host provides the guest with
>  	  timing infrastructure such as time of day, and system time
>  
> +config ARCH_CPUIDLE_HALTPOLL
> +        def_bool n
> +        proUmpt "Disable host haltpoll when loading haltpoll driver"
> +        help
> +	  If virtualized under KVM, disable host haltpoll.
> +
>  config PVH
>  	bool "Support for running PVH guests"
>  	---help---
> Index: linux-2.6.git/arch/x86/kernel/kvm.c
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/kernel/kvm.c	2019-06-03 19:38:12.376619124 -0300
> +++ linux-2.6.git/arch/x86/kernel/kvm.c	2019-06-03 19:40:14.359024312 -0300
> @@ -853,3 +853,43 @@
>  }
>  
>  #endif	/* CONFIG_PARAVIRT_SPINLOCKS */
> +
> +#ifdef CONFIG_ARCH_CPUIDLE_HALTPOLL
> +
> +void kvm_disable_host_haltpoll(void *i)
> +{
> +	wrmsrl(MSR_KVM_POLL_CONTROL, 0);
> +}
> +
> +void kvm_enable_host_haltpoll(void *i)
> +{
> +	wrmsrl(MSR_KVM_POLL_CONTROL, 1);
> +}
> +
> +void arch_haltpoll_enable(void)
> +{
> +	if (!kvm_para_has_feature(KVM_FEATURE_POLL_CONTROL))
> +		return;
> +

Perhaps warn the user when failing to disable host poll e.g.:

if (!kvm_para_has_feature(KVM_FEATURE_POLL_CONTROL)) {
	pr_warn_once("haltpoll: Failed to disable host halt polling\n");
	return;
}

But I wonder whether we should fail to load cpuidle-haltpoll when host halt
polling can't be disabled[*]? That is to avoid polling in both host and guest
and *possibly* avoid chances for performance regressions when running on older
hypervisors?

[*] with guest still able load with lack of host polling control via modparam

> +	preempt_disable();
> +	/* Enabling guest halt poll disables host halt poll */
> +	kvm_disable_host_haltpoll(NULL);
> +	smp_call_function(kvm_disable_host_haltpoll, NULL, 1);
> +	preempt_enable();
> +}
> +EXPORT_SYMBOL_GPL(arch_haltpoll_enable);
> +
> +void arch_haltpoll_disable(void)
> +{
> +	if (!kvm_para_has_feature(KVM_FEATURE_POLL_CONTROL))
> +		return;
> +
> +	preempt_disable();
> +	/* Enabling guest halt poll disables host halt poll */
> +	kvm_enable_host_haltpoll(NULL);
> +	smp_call_function(kvm_enable_host_haltpoll, NULL, 1);
> +	preempt_enable();
> +}
> +
> +EXPORT_SYMBOL_GPL(arch_haltpoll_disable);
> +#endif
>