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SubjectRe: [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@linux.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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