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Date: Thu, 23 May 2013 15:39:47 +0300
From: Gleb Natapov <gleb@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 Thu, May 23, 2013 at 07:13:58PM +0800, Xiao Guangrong wrote:
> 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);
Why not drop this
> 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);
and this.
Since we check for obsolete while searching hashtable why delete it
here?
> } else {
> list_move(&sp->link, &kvm->arch.active_mmu_pages);
> kvm_reload_remote_mmus(kvm);
>
> isn't it?
--
Gleb.
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