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

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Date:	Fri, 23 Dec 2011 12:11:22 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 12/23/2011 12:06 AM, Marcelo Tosatti wrote:

> 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?
> 


Actually, this function is only used to see whether the sp has unsync children
in current code.

And this patch did not change the logic, it just use sp_unsync_children_num
instead of using sp->unsync_children directly.

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