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Message-ID: <51A3E54C.5040504@linux.vnet.ibm.com>
Date: Tue, 28 May 2013 06:59:24 +0800
From: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To: Marcelo Tosatti <mtosatti@...hat.com>
CC: Gleb Natapov <gleb@...hat.com>, avi.kivity@...il.com,
pbonzini@...hat.com, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Subject: Re: [PATCH v7 03/11] KVM: MMU: fast invalidate all pages
On 05/27/2013 04:37 AM, Marcelo Tosatti wrote:
> On Sun, May 26, 2013 at 11:26:49AM +0300, Gleb Natapov wrote:
>> On Fri, May 24, 2013 at 05:23:07PM -0300, Marcelo Tosatti wrote:
>>> Hi Xiao,
>>>
>>> On Thu, May 23, 2013 at 03:55:52AM +0800, Xiao Guangrong wrote:
>>>> The current kvm_mmu_zap_all is really slow - it is holding mmu-lock to
>>>> walk and zap all shadow pages one by one, also it need to zap all guest
>>>> page's rmap and all shadow page's parent spte list. Particularly, things
>>>> become worse if guest uses more memory or vcpus. It is not good for
>>>> scalability
>>>>
>>>> In this patch, we introduce a faster way to invalidate all shadow pages.
>>>> KVM maintains a global mmu invalid generation-number which is stored in
>>>> kvm->arch.mmu_valid_gen and every shadow page stores the current global
>>>> generation-number into sp->mmu_valid_gen when it is created
>>>>
>>>> When KVM need zap all shadow pages sptes, it just simply increase the
>>>> global generation-number then reload root shadow pages on all vcpus.
>>>> Vcpu will create a new shadow page table according to current kvm's
>>>> generation-number. It ensures the old pages are not used any more.
>>>> Then the obsolete pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
>>>> are zapped by using lock-break technique
>>>>
>>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>>> ---
>>>> arch/x86/include/asm/kvm_host.h | 2 +
>>>> arch/x86/kvm/mmu.c | 84 +++++++++++++++++++++++++++++++++++++++
>>>> arch/x86/kvm/mmu.h | 1 +
>>>> 3 files changed, 87 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>> index 3741c65..bff7d46 100644
>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>> @@ -222,6 +222,7 @@ struct kvm_mmu_page {
>>>> int root_count; /* Currently serving as active root */
>>>> unsigned int unsync_children;
>>>> unsigned long parent_ptes; /* Reverse mapping for parent_pte */
>>>> + unsigned long mmu_valid_gen;
>>>> DECLARE_BITMAP(unsync_child_bitmap, 512);
>>>>
>>>> #ifdef CONFIG_X86_32
>>>> @@ -529,6 +530,7 @@ struct kvm_arch {
>>>> unsigned int n_requested_mmu_pages;
>>>> unsigned int n_max_mmu_pages;
>>>> unsigned int indirect_shadow_pages;
>>>> + unsigned long mmu_valid_gen;
>>>> struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
>>>> /*
>>>> * Hash table of struct kvm_mmu_page.
>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>> index f8ca2f3..f302540 100644
>>>> --- a/arch/x86/kvm/mmu.c
>>>> +++ b/arch/x86/kvm/mmu.c
>>>> @@ -1838,6 +1838,11 @@ 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,
>>>> @@ -1900,6 +1905,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>>>>
>>>> account_shadowed(vcpu->kvm, gfn);
>>>> }
>>>> + sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;
>>>> init_shadow_page_table(sp);
>>>> trace_kvm_mmu_get_page(sp, true);
>>>> return sp;
>>>> @@ -2070,8 +2076,10 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>>>> ret = mmu_zap_unsync_children(kvm, sp, invalid_list);
>>>> kvm_mmu_page_unlink_children(kvm, sp);
>>>> kvm_mmu_unlink_parents(kvm, sp);
>>>> +
>>>> if (!sp->role.invalid && !sp->role.direct)
>>>> unaccount_shadowed(kvm, sp->gfn);
>>>> +
>>>> if (sp->unsync)
>>>> kvm_unlink_unsync_page(kvm, sp);
>>>> if (!sp->root_count) {
>>>> @@ -4195,6 +4203,82 @@ restart:
>>>> spin_unlock(&kvm->mmu_lock);
>>>> }
>>>>
>>>> +static void kvm_zap_obsolete_pages(struct kvm *kvm)
>>>> +{
>>>> + struct kvm_mmu_page *sp, *node;
>>>> + LIST_HEAD(invalid_list);
>>>> +
>>>> +restart:
>>>> + list_for_each_entry_safe_reverse(sp, node,
>>>> + &kvm->arch.active_mmu_pages, link) {
>>>> + /*
>>>> + * No obsolete page exists before new created page since
>>>> + * active_mmu_pages is the FIFO list.
>>>> + */
>>>> + if (!is_obsolete_sp(kvm, sp))
>>>> + break;
>>>
>>> Can you add a comment to list_add(x, active_mmu_pages) callsites
>>> mentioning this case?
>>>
>>> Because it'll break silently if people do list_add_tail().
>>>
>>>> + /*
>>>> + * Do not repeatedly zap a root page to avoid unnecessary
>>>> + * KVM_REQ_MMU_RELOAD, otherwise we may not be able to
>>>> + * progress:
>>>> + * vcpu 0 vcpu 1
>>>> + * call vcpu_enter_guest():
>>>> + * 1): handle KVM_REQ_MMU_RELOAD
>>>> + * and require mmu-lock to
>>>> + * load mmu
>>>> + * repeat:
>>>> + * 1): zap root page and
>>>> + * send KVM_REQ_MMU_RELOAD
>>>> + *
>>>> + * 2): if (cond_resched_lock(mmu-lock))
>>>> + *
>>>> + * 2): hold mmu-lock and load mmu
>>>> + *
>>>> + * 3): see KVM_REQ_MMU_RELOAD bit
>>>> + * on vcpu->requests is set
>>>> + * then return 1 to call
>>>> + * vcpu_enter_guest() again.
>>>> + * goto repeat;
>>>> + *
>>>> + * Since we are reversely walking the list and the invalid
>>>> + * list will be moved to the head, skip the invalid page
>>>> + * can help us to avoid the infinity list walking.
>>>> + */
>>>> + if (sp->role.invalid)
>>>> + continue;
>>>
>>> But this allows completing (that is returning), with page that should
>>> be zapped still present (even though its invalid).
>>>
>>> Is another pass needed at the end to take care of the invalid pages?
>>> Which at that point must have their root_count decreased.
>>>
>> It is not different from how it work now. Invalid page can still be not
>> zapped after zap_all() completes. They are ignored by all relevant code
>> paths.
>
> It is different. kvm_mmu_zap_all() returns with 0 pages at
> mmu_active_pages today.
Do not think so.
kvm_mmu_zap_all() also keeps role.root && role.invalid pages on mmu_active_pages.
role.root && role.invalid pages should be zapped in vcpu context.
>
> It is probably worthwhile to maintain behaviour (instead of every other
> code path does the right thing).
>
>>> Also this function should be serialized, that is, should not allow
>>> simultaneous kvm_mmu_invalidate_zap_all_pages. If thats so
>>> assert(mutex_is_locked(kvm->lock)) would help.
>>>
>>> Probably fine to have simultaneous users, but not necessary AFAICS.
>> I raced this point on previous patch submission. The function can be
>> executed simultaneously only during vmexit when zap_all is called by mm
>> notifiers. During regular work the function is only called by slot
>> manipulation functions and those are serialized by slot lock.
>
> How about fix hypercall vs kvm_set_memory?
In this version, we have dropped call kvm_mmu_zap_all in hypercall since
it is useless.
(pacthing hypercall is broken for a long time:
https://lkml.org/lkml/2013/5/22/388)
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