linux-kernel - Re: [PATCH 1/8] KVM: MMU: combine unsync and unsync

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Date:	Thu, 22 Dec 2011 14:06:41 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/8] KVM: MMU: combine unsync and unsync_children

On Fri, Dec 16, 2011 at 06:13:42PM +0800, Xiao Guangrong wrote:
> unsync is only used for the page of level == 1 and unsync_children is only
> used in the upper page, so combine them to reduce the size of shadow page
> structure
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  Documentation/virtual/kvm/mmu.txt |    5 ++-
>  arch/x86/include/asm/kvm_host.h   |   14 +++++++++++-
>  arch/x86/kvm/mmu.c                |   39 +++++++++++++++++++++++++-----------
>  arch/x86/kvm/mmu_audit.c          |    6 ++--
>  arch/x86/kvm/mmutrace.h           |    3 +-
>  arch/x86/kvm/paging_tmpl.h        |    2 +-
>  6 files changed, 48 insertions(+), 21 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
> index 5dc972c..4a5fedd 100644
> --- a/Documentation/virtual/kvm/mmu.txt
> +++ b/Documentation/virtual/kvm/mmu.txt
> @@ -205,14 +205,15 @@ Shadow pages contain the following information:
>      this page's spt.  Otherwise, parent_ptes points at a data structure
>      with a list of parent_ptes.
>    unsync:
> +    It is used when role.level == 1, only level 1 shadow pages can be unsync.
>      If true, then the translations in this page may not match the guest's
>      translation.  This is equivalent to the state of the tlb when a pte is
>      changed but before the tlb entry is flushed.  Accordingly, unsync ptes
>      are synchronized when the guest executes invlpg or flushes its tlb by
>      other means.  Valid for leaf pages.
>    unsync_children:
> -    How many sptes in the page point at pages that are unsync (or have
> -    unsynchronized children).
> +    It is used when role.level > 1 and indicates how many sptes in the page
> +    point at unsync pages or unsynchronized children.
>    unsync_child_bitmap:
>      A bitmap indicating which sptes in spt point (directly or indirectly) at
>      pages that may be unsynchronized.  Used to quickly locate all unsychronized
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 52d6640..c0a89cd 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -233,9 +233,19 @@ struct kvm_mmu_page {
>  	 * in this shadow page.
>  	 */
>  	DECLARE_BITMAP(slot_bitmap, KVM_MEM_SLOTS_NUM);
> -	bool unsync;
>  	int root_count;          /* Currently serving as active root */
> -	unsigned int unsync_children;
> +
> +	/*
> +	 * If role.level == 1, unsync indicates whether the sp is
> +	 * unsync, otherwise unsync_children indicates the number
> +	 * of sptes which point at unsync sp or unsychronized children.
> +	 * See sp_is_unsync() / sp_unsync_children_num.
> +	 */
> +	union {
> +		bool unsync;
> +		unsigned int unsync_children;
> +	};
> +
>  	unsigned long parent_ptes;	/* Reverse mapping for parent_pte */
>  	DECLARE_BITMAP(unsync_child_bitmap, 512);
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 2a2a9b4..6913a16 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -151,6 +151,21 @@ module_param(dbg, bool, 0644);
> 
>  #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> 
> +static bool sp_is_unsync(struct kvm_mmu_page *sp)
> +{
> +	return sp->role.level == PT_PAGE_TABLE_LEVEL && sp->unsync;
> +}
> +
> +static unsigned int sp_unsync_children_num(struct kvm_mmu_page *sp)
> +{
> +	unsigned int num = 0;
> +
> +	if (sp->role.level != PT_PAGE_TABLE_LEVEL)
> +		num = sp->unsync_children;
> +
> +	return num;
> +}

IIRC there are windows where unsync_children is not accurate. Have you 
verified this function is not called inside one of those windows?

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