Re: [PATCH] mm/x86: Patch out arch_flush_lazy_mmu_mode() when running on bare metal

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Feb 28, 2013 at 05:55:49PM -0500, Boris Ostrovsky wrote:
> Invoking arch_flush_lazy_mmu_mode() results in calls to
> preempt_enable()/disable() which may have performance impact.
> 
> Since lazy MMU is not used on bare metal we can patch away
> arch_flush_lazy_mmu_mode() so that it is never called in such
> environment.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

I am OK with this  - had also run it overnight on baremetal/xen to make
sure there were no funny business.

> ---
>  arch/x86/include/asm/paravirt.h       |  5 ++++-
>  arch/x86/include/asm/paravirt_types.h |  2 ++
>  arch/x86/kernel/paravirt.c            | 25 +++++++++++++------------
>  arch/x86/lguest/boot.c                |  1 +
>  arch/x86/xen/mmu.c                    |  1 +
>  5 files changed, 21 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 5edd174..7361e47 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -703,7 +703,10 @@ static inline void arch_leave_lazy_mmu_mode(void)
>  	PVOP_VCALL0(pv_mmu_ops.lazy_mode.leave);
>  }
>  
> -void arch_flush_lazy_mmu_mode(void);
> +static inline void arch_flush_lazy_mmu_mode(void)
> +{
> +	PVOP_VCALL0(pv_mmu_ops.lazy_mode.flush);
> +}
>  
>  static inline void __set_fixmap(unsigned /* enum fixed_addresses */ idx,
>  				phys_addr_t phys, pgprot_t flags)
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index 142236e..b3b0ec1 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -91,6 +91,7 @@ struct pv_lazy_ops {
>  	/* Set deferred update mode, used for batching operations. */
>  	void (*enter)(void);
>  	void (*leave)(void);
> +	void (*flush)(void);
>  };
>  
>  struct pv_time_ops {
> @@ -679,6 +680,7 @@ void paravirt_end_context_switch(struct task_struct *next);
>  
>  void paravirt_enter_lazy_mmu(void);
>  void paravirt_leave_lazy_mmu(void);
> +void paravirt_flush_lazy_mmu(void);
>  
>  void _paravirt_nop(void);
>  u32 _paravirt_ident_32(u32);
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index 17fff18..8bfb335 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -263,6 +263,18 @@ void paravirt_leave_lazy_mmu(void)
>  	leave_lazy(PARAVIRT_LAZY_MMU);
>  }
>  
> +void paravirt_flush_lazy_mmu(void)
> +{
> +	preempt_disable();
> +
> +	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
> +		arch_leave_lazy_mmu_mode();
> +		arch_enter_lazy_mmu_mode();
> +	}
> +
> +	preempt_enable();
> +}
> +
>  void paravirt_start_context_switch(struct task_struct *prev)
>  {
>  	BUG_ON(preemptible());
> @@ -292,18 +304,6 @@ enum paravirt_lazy_mode paravirt_get_lazy_mode(void)
>  	return this_cpu_read(paravirt_lazy_mode);
>  }
>  
> -void arch_flush_lazy_mmu_mode(void)
> -{
> -	preempt_disable();
> -
> -	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
> -		arch_leave_lazy_mmu_mode();
> -		arch_enter_lazy_mmu_mode();
> -	}
> -
> -	preempt_enable();
> -}
> -
>  struct pv_info pv_info = {
>  	.name = "bare hardware",
>  	.paravirt_enabled = 0,
> @@ -475,6 +475,7 @@ struct pv_mmu_ops pv_mmu_ops = {
>  	.lazy_mode = {
>  		.enter = paravirt_nop,
>  		.leave = paravirt_nop,
> +		.flush = paravirt_nop,
>  	},
>  
>  	.set_fixmap = native_set_fixmap,
> diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
> index 1cbd89c..7114c63 100644
> --- a/arch/x86/lguest/boot.c
> +++ b/arch/x86/lguest/boot.c
> @@ -1334,6 +1334,7 @@ __init void lguest_init(void)
>  	pv_mmu_ops.read_cr3 = lguest_read_cr3;
>  	pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu;
>  	pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mmu_mode;
> +	pv_mmu_ops.lazy_mode.flush = paravirt_flush_lazy_mmu;
>  	pv_mmu_ops.pte_update = lguest_pte_update;
>  	pv_mmu_ops.pte_update_defer = lguest_pte_update;
>  
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index e8e3493..f4f4105 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -2197,6 +2197,7 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst = {
>  	.lazy_mode = {
>  		.enter = paravirt_enter_lazy_mmu,
>  		.leave = xen_leave_lazy_mmu,
> +		.flush = paravirt_flush_lazy_mmu,
>  	},
>  
>  	.set_fixmap = xen_set_fixmap,
> -- 
> 1.8.1.2
>