linux-kernel - Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

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Date:	Sat, 04 May 2013 00:51:06 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	gleb@...hat.com, avi.kivity@...il.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	takuya.yoshikawa@...il.com
Subject: Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

On 05/03/2013 11:53 PM, Marcelo Tosatti wrote:
> On Fri, May 03, 2013 at 01:52:07PM +0800, Xiao Guangrong wrote:
>> On 05/03/2013 09:05 AM, Marcelo Tosatti wrote:
>>
>>>> +
>>>> +/*
>>>> + * Fast invalid all shadow pages belong to @slot.
>>>> + *
>>>> + * @slot != NULL means the invalidation is caused the memslot specified
>>>> + * by @slot is being deleted, in this case, we should ensure that rmap
>>>> + * and lpage-info of the @slot can not be used after calling the function.
>>>> + *
>>>> + * @slot == NULL means the invalidation due to other reasons, we need
>>>> + * not care rmap and lpage-info since they are still valid after calling
>>>> + * the function.
>>>> + */
>>>> +void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
>>>> +				   struct kvm_memory_slot *slot)
>>>> +{
>>>> +	spin_lock(&kvm->mmu_lock);
>>>> +	kvm->arch.mmu_valid_gen++;
>>>> +
>>>> +	/*
>>>> +	 * All shadow paes are invalid, reset the large page info,
>>>> +	 * then we can safely desotry the memslot, it is also good
>>>> +	 * for large page used.
>>>> +	 */
>>>> +	kvm_clear_all_lpage_info(kvm);
>>>
>>> Xiao,
>>>
>>> I understood it was agreed that simple mmu_lock lockbreak while
>>> avoiding zapping of newly instantiated pages upon a
>>>
>>> 	if(spin_needbreak)
>>> 		cond_resched_lock()
>>>
>>> cycle was enough as a first step? And then later introduce root zapping
>>> along with measurements.
>>>
>>> https://lkml.org/lkml/2013/4/22/544
>>
>> Yes, it is.
>>
>> See the changelog in 0/0:
>>
>> " we use lock-break technique to zap all sptes linked on the
>> invalid rmap, it is not very effective but good for the first step."
>>
>> Thanks!
> 
> Sure, but what is up with zeroing kvm_clear_all_lpage_info(kvm) and
> zapping the root? Only lock-break technique along with generation number 
> was what was agreed.

Marcelo,

Please Wait... I am completely confused. :(

Let's clarify "zeroing kvm_clear_all_lpage_info(kvm) and zapping the root" first.
Are these changes you wanted?

void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
				   struct kvm_memory_slot *slot)
{
	spin_lock(&kvm->mmu_lock);
	kvm->arch.mmu_valid_gen++;

	/* Zero all root pages.*/
restart:
	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
		if (!sp->root_count)
			continue;

		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
			goto restart;
	}

	/*
	 * All shadow paes are invalid, reset the large page info,
	 * then we can safely desotry the memslot, it is also good
	 * for large page used.
	 */
	kvm_clear_all_lpage_info(kvm);

	kvm_mmu_commit_zap_page(kvm, &invalid_list);
	spin_unlock(&kvm->mmu_lock);
}

static void rmap_remove(struct kvm *kvm, u64 *spte)
{
	struct kvm_mmu_page *sp;
	gfn_t gfn;
	unsigned long *rmapp;

	sp = page_header(__pa(spte));
+
+       /* Let invalid sp do not access its rmap. */
+	if (!sp_is_valid(sp))
+		return;
+
	gfn = kvm_mmu_page_get_gfn(sp, spte - sp->spt);
	rmapp = gfn_to_rmap(kvm, gfn, sp->role.level);
	pte_list_remove(spte, rmapp);
}

If yes, there is the reason why we can not do this that i mentioned before:

after call kvm_mmu_invalid_memslot_pages(), the memslot->rmap will be destroyed.
Later, if host reclaim page, the mmu-notify handlers, ->invalidate_page and
->invalidate_range_start, can not find any spte using the host page, then
Accessed/Dirty for host page is missing tracked.
(missing call kvm_set_pfn_accessed and kvm_set_pfn_dirty properly.)

What's your idea?

And I should apologize for my poor communications, really sorry for that...


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