Re: [PATCH v9 4/4] arm: ARMv7 dirty page logging 2nd stage page fault handling support

[Mario Smarduch <m.smarduch@samsung.com>]

On 08/12/2014 02:50 AM, Christoffer Dall wrote:
> On Mon, Aug 11, 2014 at 06:25:05PM -0700, Mario Smarduch wrote:
>> On 08/11/2014 12:13 PM, Christoffer Dall wrote:
>>> On Thu, Jul 24, 2014 at 05:56:08PM -0700, Mario Smarduch wrote:
>>>> This patch adds support for handling 2nd stage page faults during migration,
>>>> it disables faulting in huge pages, and dissolves huge pages to page tables.
>>>> In case migration is canceled huge pages will be used again.
>>>>
>>>> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
>>>> ---
>>>>  arch/arm/kvm/mmu.c |   31 +++++++++++++++++++++++++------
>>>>  1 file changed, 25 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index ca84331..a17812a 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -642,7 +642,8 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
>>>>  }
>>>>  
>>>>  static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>>> -			  phys_addr_t addr, const pte_t *new_pte, bool iomap)
>>>> +			  phys_addr_t addr, const pte_t *new_pte, bool iomap,
>>>> +			  bool logging_active)
>>>>  {
>>>>  	pmd_t *pmd;
>>>>  	pte_t *pte, old_pte;
>>>> @@ -657,6 +658,15 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>>>  		return 0;
>>>>  	}
>>>>  
>>>> +	/*
>>>> +	 * While dirty memory logging, clear PMD entry for huge page and split
>>>> +	 * into smaller pages, to track dirty memory at page granularity.
>>>> +	 */
>>>> +	if (logging_active && kvm_pmd_huge(*pmd)) {
>>>> +		phys_addr_t ipa = pmd_pfn(*pmd) << PAGE_SHIFT;
>>>> +		clear_pmd_entry(kvm, pmd, ipa);
>>>
>>> clear_pmd_entry has a VM_BUG_ON(kvm_pmd_huge(*pmd)) so that is
>>> definitely not the right thing to call.
>>
>> I don't see that in 3.15rc1/rc4 -
>>
>> static void clear_pmd_entry(struct kvm *kvm, pmd_t *pmd, phys_addr_t addr)
>> {
>>         if (kvm_pmd_huge(*pmd)) {
>>                 pmd_clear(pmd);
>>                 kvm_tlb_flush_vmid_ipa(kvm, addr);
>>         } else {
>>                   [....]
>> }
>>
>> I thought the purpose of this function was to clear PMD entry. Also
>> ran hundreds of tests no problems. Hmmm confused.
>>
> 
> You need to rebase on kvm/next or linus/master, something that contains:
> 
> 4f853a7 arm/arm64: KVM: Fix and refactor unmap_range
> 
>>>
>>>> +	}
>>>> +
>>>>  	/* Create stage-2 page mappings - Level 2 */
>>>>  	if (pmd_none(*pmd)) {
>>>>  		if (!cache)
>>>> @@ -709,7 +719,7 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
>>>>  		if (ret)
>>>>  			goto out;
>>>>  		spin_lock(&kvm->mmu_lock);
>>>> -		ret = stage2_set_pte(kvm, &cache, addr, &pte, true);
>>>> +		ret = stage2_set_pte(kvm, &cache, addr, &pte, true, false);
>>>>  		spin_unlock(&kvm->mmu_lock);
>>>>  		if (ret)
>>>>  			goto out;
>>>> @@ -926,6 +936,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  	struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
>>>>  	struct vm_area_struct *vma;
>>>>  	pfn_t pfn;
>>>> +	/* Get logging status, if dirty_bitmap is not NULL then logging is on */
>>>> +	#ifdef CONFIG_ARM
>>>> +		bool logging_active = !!memslot->dirty_bitmap;
>>>> +	#else
>>>> +		bool logging_active = false;
>>>> +	#endif
>>>
>>> can you make this an inline in the header files for now please?
>>
>> Yes definitely.
>>
>>>
>>>>  
>>>>  	write_fault = kvm_is_write_fault(kvm_vcpu_get_hsr(vcpu));
>>>>  	if (fault_status == FSC_PERM && !write_fault) {
>>>> @@ -936,7 +952,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  	/* Let's check if we will get back a huge page backed by hugetlbfs */
>>>>  	down_read(&current->mm->mmap_sem);
>>>>  	vma = find_vma_intersection(current->mm, hva, hva + 1);
>>>> -	if (is_vm_hugetlb_page(vma)) {
>>>> +	if (is_vm_hugetlb_page(vma) && !logging_active) {
>>>>  		hugetlb = true;
>>>>  		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>>>>  	} else {
>>>> @@ -979,7 +995,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  	spin_lock(&kvm->mmu_lock);
>>>>  	if (mmu_notifier_retry(kvm, mmu_seq))
>>>>  		goto out_unlock;
>>>> -	if (!hugetlb && !force_pte)
>>>> +	if (!hugetlb && !force_pte && !logging_active)
>>>>  		hugetlb = transparent_hugepage_adjust(&pfn, &fault_ipa);
>>>>  
>>>>  	if (hugetlb) {
>>>> @@ -998,9 +1014,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  			kvm_set_pfn_dirty(pfn);
>>>>  		}
>>>>  		coherent_cache_guest_page(vcpu, hva, PAGE_SIZE);
>>>> -		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, false);
>>>> +		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, false,
>>>> +					logging_active);
>>>>  	}
>>>>  
>>>> +	if (write_fault)
>>>> +		mark_page_dirty(kvm, gfn);
>>>>  
>>>>  out_unlock:
>>>>  	spin_unlock(&kvm->mmu_lock);
>>>> @@ -1151,7 +1170,7 @@ static void kvm_set_spte_handler(struct kvm *kvm, gpa_t gpa, void *data)
>>>>  {
>>>>  	pte_t *pte = (pte_t *)data;
>>>>  
>>>> -	stage2_set_pte(kvm, NULL, gpa, pte, false);
>>>> +	stage2_set_pte(kvm, NULL, gpa, pte, false, false);
>>>
>>> why is logging never active if we are called from MMU notifiers?
>>
>> mmu notifiers update sptes, but I don't see how these updates
>> can result in guest dirty pages. Also guest pages are marked dirty
>> from 2nd stage page fault handlers (searching through the code).
>>
> Ok, then add:
> 
> /*
>  * We can always call stage2_set_pte with logging_active == false,
>  * because MMU notifiers will have unmapped a huge PMD before calling
>  * ->change_pte() (which in turn calls kvm_set_spte_hva()) and therefore
>  * stage2_set_pte() never needs to clear out a huge PMD through this
>  * calling path.
>  */

So here on permission change to primary pte's kernel first invalidates
related s2ptes followed by ->change_pte calls to synchronize s2ptes. As
consequence of invalidation we unmap huge PMDs, if a page falls in that
range.

Is the comment to point out use of logging flag under various scenarios?

Should I add comments on flag use in other places as well?

> 
> Thanks,
> -Christoffer
>