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Date: Thu, 23 May 2013 19:13:58 +0800
From: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To: Gleb Natapov <gleb@...hat.com>
CC: avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 09/11] KVM: MMU: introduce kvm_mmu_prepare_zap_obsolete_page
On 05/23/2013 04:09 PM, Gleb Natapov wrote:
> On Thu, May 23, 2013 at 03:50:16PM +0800, Xiao Guangrong wrote:
>> On 05/23/2013 03:37 PM, Gleb Natapov wrote:
>>> On Thu, May 23, 2013 at 02:31:47PM +0800, Xiao Guangrong wrote:
>>>> On 05/23/2013 02:18 PM, Gleb Natapov wrote:
>>>>> On Thu, May 23, 2013 at 02:13:06PM +0800, Xiao Guangrong wrote:
>>>>>> On 05/23/2013 01:57 PM, Gleb Natapov wrote:
>>>>>>> On Thu, May 23, 2013 at 03:55:58AM +0800, Xiao Guangrong wrote:
>>>>>>>> It is only used to zap the obsolete page. Since the obsolete page
>>>>>>>> will not be used, we need not spend time to find its unsync children
>>>>>>>> out. Also, we delete the page from shadow page cache so that the page
>>>>>>>> is completely isolated after call this function.
>>>>>>>>
>>>>>>>> The later patch will use it to collapse tlb flushes
>>>>>>>>
>>>>>>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>>>>>>> ---
>>>>>>>> arch/x86/kvm/mmu.c | 46 +++++++++++++++++++++++++++++++++++++++++-----
>>>>>>>> 1 files changed, 41 insertions(+), 5 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>>>>>> index 9b57faa..e676356 100644
>>>>>>>> --- a/arch/x86/kvm/mmu.c
>>>>>>>> +++ b/arch/x86/kvm/mmu.c
>>>>>>>> @@ -1466,7 +1466,7 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
>>>>>>>> static void kvm_mmu_free_page(struct kvm_mmu_page *sp)
>>>>>>>> {
>>>>>>>> ASSERT(is_empty_shadow_page(sp->spt));
>>>>>>>> - hlist_del(&sp->hash_link);
>>>>>>>> + hlist_del_init(&sp->hash_link);
>>>>>>> Why do you need hlist_del_init() here? Why not move it into
>>>>>>
>>>>>> Since the hlist will be double freed. We will it like this:
>>>>>>
>>>>>> kvm_mmu_prepare_zap_obsolete_page(page, list);
>>>>>> kvm_mmu_commit_zap_page(list);
>>>>>> kvm_mmu_free_page(page);
>>>>>>
>>>>>> The first place is kvm_mmu_prepare_zap_obsolete_page(page), which have
>>>>>> deleted the hash list.
>>>>>>
>>>>>>> kvm_mmu_prepare_zap_page() like we discussed it here:
>>>>>>> https://patchwork.kernel.org/patch/2580351/ instead of doing
>>>>>>> it differently for obsolete and non obsolete pages?
>>>>>>
>>>>>> It is can break the hash-list walking: we should rescan the
>>>>>> hash list once the page is prepared-ly zapped.
>>>>>>
>>>>>> I mentioned it in the changelog:
>>>>>>
>>>>>> 4): drop the patch which deleted page from hash list at the "prepare"
>>>>>> time since it can break the walk based on hash list.
>>>>> Can you elaborate on how this can happen?
>>>>
>>>> There is a example:
>>>>
>>>> int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
>>>> {
>>>> struct kvm_mmu_page *sp;
>>>> LIST_HEAD(invalid_list);
>>>> int r;
>>>>
>>>> pgprintk("%s: looking for gfn %llx\n", __func__, gfn);
>>>> r = 0;
>>>> spin_lock(&kvm->mmu_lock);
>>>> for_each_gfn_indirect_valid_sp(kvm, sp, gfn) {
>>>> pgprintk("%s: gfn %llx role %x\n", __func__, gfn,
>>>> sp->role.word);
>>>> r = 1;
>>>> kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
>>>> }
>>>> kvm_mmu_commit_zap_page(kvm, &invalid_list);
>>>> spin_unlock(&kvm->mmu_lock);
>>>>
>>>> return r;
>>>> }
>>>>
>>>> It works fine since kvm_mmu_prepare_zap_page does not touch the hash list.
>>>> If we delete hlist in kvm_mmu_prepare_zap_page(), this kind of codes should
>>>> be changed to:
>>>>
>>>> restart:
>>>> for_each_gfn_indirect_valid_sp(kvm, sp, gfn) {
>>>> pgprintk("%s: gfn %llx role %x\n", __func__, gfn,
>>>> sp->role.word);
>>>> r = 1;
>>>> if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
>>>> goto restart;
>>>> }
>>>> kvm_mmu_commit_zap_page(kvm, &invalid_list);
>>>>
>>> Hmm, yes. So lets leave it as is and always commit invalid_list before
>>
>> So, you mean drop this patch and the patch of
>> KVM: MMU: collapse TLB flushes when zap all pages?
>>
> We still want to add kvm_reload_remote_mmus() to
> kvm_mmu_invalidate_zap_all_pages(). But yes, we disable a nice
> optimization here. So may be skipping obsolete pages while walking
> hashtable is better solution.
I am willing to use this way instead, but it looks worse than this
patch:
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9b57faa..810410c 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1466,7 +1466,7 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
static void kvm_mmu_free_page(struct kvm_mmu_page *sp)
{
ASSERT(is_empty_shadow_page(sp->spt));
- hlist_del(&sp->hash_link);
+ hlist_del_init(&sp->hash_link);
list_del(&sp->link);
free_page((unsigned long)sp->spt);
if (!sp->role.direct)
@@ -1648,14 +1648,20 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
static void kvm_mmu_commit_zap_page(struct kvm *kvm,
struct list_head *invalid_list);
+static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
+{
+ return unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
+}
+
#define for_each_gfn_sp(_kvm, _sp, _gfn) \
hlist_for_each_entry(_sp, \
&(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)], hash_link) \
- if ((_sp)->gfn != (_gfn)) {} else
+ if ((_sp)->gfn != (_gfn) || is_obsolete_sp(_kvm, _sp)) {} else
#define for_each_gfn_indirect_valid_sp(_kvm, _sp, _gfn) \
for_each_gfn_sp(_kvm, _sp, _gfn) \
- if ((_sp)->role.direct || (_sp)->role.invalid) {} else
+ if ((_sp)->role.direct || \
+ (_sp)->role.invalid || is_obsolete_sp(_kvm, _sp)) {} else
/* @sp->gfn should be write-protected at the call site */
static int __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
@@ -1838,11 +1844,6 @@ static void clear_sp_write_flooding_count(u64 *spte)
__clear_sp_write_flooding_count(sp);
}
-static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
-{
- return unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
-}
-
static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
gfn_t gfn,
gva_t gaddr,
@@ -2085,11 +2086,15 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
if (sp->unsync)
kvm_unlink_unsync_page(kvm, sp);
+
if (!sp->root_count) {
/* Count self */
ret++;
list_move(&sp->link, invalid_list);
kvm_mod_used_mmu_pages(kvm, -1);
+
+ if (unlikely(is_obsolete_sp(kvm, sp)))
+ hlist_del_init(&sp->hash_link);
} else {
list_move(&sp->link, &kvm->arch.active_mmu_pages);
kvm_reload_remote_mmus(kvm);
isn't it?
--
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