Subject: [PATCH RESEND v15 07/10] KVM: arm: page logging 2nd stage fault handling

From: [email-masked] (Mario Smarduch)
Date: Tue, 13 Jan 2015 15:14:47 -0800

On 01/12/2015 09:49 AM, Christoffer Dall wrote:
> On Mon, Jan 12, 2015 at 08:27:03AM -0800, Mario Smarduch wrote:
>> On 01/11/2015 06:00 AM, Christoffer Dall wrote:
>>> On Fri, Jan 09, 2015 at 08:17:20PM -0800, Mario Smarduch wrote:
>>>> This patch adds support for 2nd stage page fault handling while dirty page
>>>> logging. On huge page faults, huge pages are dissolved to normal pages, and
>>>> rebuilding of 2nd stage huge pages is blocked. In case migration is 
>>>> canceled this restriction is removed and huge pages may be rebuilt again.
>>>>
>>>> This patch applies cleanly on top of patch series posted Dec. 15'th:
>>>> https://lists.cs.columbia.edu/pipermail/kvmarm/2014-December/012826.html
>>>
>>> In the future such information should also go under the ---
>>> separator.
>>>
>>>>
>>>> Patch #11 has been dropped, and should not be applied.
>>>
>>> this should go under the '---' separator too.
>> Ok will keep that in mind.
> 
> basically, think of everything above the '---' separator as the commit
> message you will find in 'git log' when you are trying to understand a
> piece of code or bisecting an issue or the like.  For those purposes you
> don't care about the mechanics of how a patch was applied, how many
> iterations of the patch there were, what changed between the iterations
> and so on.
> 
>>>
>>>>
>>>> Signed-off-by: Mario Smarduch <[email-masked]>
>>>> ---
>>>>
>>>> Change Log since last RESEND v1 --> v2:
>>>> - Disallow dirty page logging of IO region - fail for initial write protect
>>>>   and disable logging code in 2nd stage page fault handler.
>>>> - Fixed auto spell correction errors
>>>>
>>>> Change Log RESEND v0 --> v1:
>>>> - fixed bug exposed by new generic __get_user_pages_fast(), when region is 
>>>>   writable, prevent write protection of pte on read fault
>>>> - Removed marking entire huge page dirty on initial access
>>>> - don't dissolve huge pages of non-writable regions
>>>> - Made updates based on Christoffers comments
>>>>   - renamed logging status function to memslot_is_logging()
>>>>   - changed few values to bool from longs
>>>>   - streamlined user_mem_abort() to eliminate extra conditional checks
>>>> ---
>>>>  arch/arm/kvm/mmu.c |  113 ++++++++++++++++++++++++++++++++++++++++++++++++----
>>>>  1 file changed, 105 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index 73d506f..b878236 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -47,6 +47,18 @@ static phys_addr_t hyp_idmap_vector;
>>>>  #define kvm_pmd_huge(_x)	(pmd_huge(_x) || pmd_trans_huge(_x))
>>>>  #define kvm_pud_huge(_x)	pud_huge(_x)
>>>>  
>>>> +#define KVM_S2PTE_FLAG_IS_IOMAP		(1UL << 0)
>>>> +#define KVM_S2PTE_FLAG_LOGGING_ACTIVE	(1UL << 1)
>>>> +
>>>> +static bool memslot_is_logging(struct kvm_memory_slot *memslot)
>>>> +{
>>>> +#ifdef CONFIG_ARM
>>>> +	return !!memslot->dirty_bitmap;
>>>> +#else
>>>> +	return false;
>>>> +#endif
>>>> +}
>>>> +
>>>>  static void kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
>>>>  {
>>>>  	/*
>>>> @@ -59,6 +71,25 @@ static void kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
>>>>  		kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, kvm, ipa);
>>>>  }
>>>>  
>>>> +/**
>>>> + * stage2_dissolve_pmd() - clear and flush huge PMD entry
>>>> + * @kvm:	pointer to kvm structure.
>>>> + * @addr:	IPA
>>>> + * @pmd:	pmd pointer for IPA
>>>> + *
>>>> + * Function clears a PMD entry, flushes addr 1st and 2nd stage TLBs. Marks all
>>>> + * pages in the range dirty.
>>>> + */
>>>> +static void stage2_dissolve_pmd(struct kvm *kvm, phys_addr_t addr, pmd_t *pmd)
>>>> +{
>>>> +	if (!kvm_pmd_huge(*pmd))
>>>> +		return;
>>>> +
>>>> +	pmd_clear(pmd);
>>>> +	kvm_tlb_flush_vmid_ipa(kvm, addr);
>>>> +	put_page(virt_to_page(pmd));
>>>> +}
>>>> +
>>>>  static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
>>>>  				  int min, int max)
>>>>  {
>>>> @@ -703,10 +734,13 @@ 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,
>>>> +			  unsigned long flags)
>>>>  {
>>>>  	pmd_t *pmd;
>>>>  	pte_t *pte, old_pte;
>>>> +	bool iomap = flags & KVM_S2PTE_FLAG_IS_IOMAP;
>>>> +	bool logging_active = flags & KVM_S2PTE_FLAG_LOGGING_ACTIVE;
>>>>  
>>>>  	/* Create stage-2 page table mapping - Levels 0 and 1 */
>>>>  	pmd = stage2_get_pmd(kvm, cache, addr);
>>>> @@ -718,6 +752,13 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>>>  		return 0;
>>>>  	}
>>>>  
>>>> +	/*
>>>> +	 * While dirty page logging - dissolve huge PMD, then continue on to
>>>> +	 * allocate page.
>>>> +	 */
>>>> +	if (logging_active)
>>>> +		stage2_dissolve_pmd(kvm, addr, pmd);
>>>> +
>>>>  	/* Create stage-2 page mappings - Level 2 */
>>>>  	if (pmd_none(*pmd)) {
>>>>  		if (!cache)
>>>> @@ -774,7 +815,8 @@ 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,
>>>> +						KVM_S2PTE_FLAG_IS_IOMAP);
>>>>  		spin_unlock(&kvm->mmu_lock);
>>>>  		if (ret)
>>>>  			goto out;
>>>> @@ -1002,6 +1044,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  	pfn_t pfn;
>>>>  	pgprot_t mem_type = PAGE_S2;
>>>>  	bool fault_ipa_uncached;
>>>> +	bool can_set_pte_rw = true;
>>>> +	unsigned long set_pte_flags = 0;
>>>>  
>>>>  	write_fault = kvm_is_write_fault(vcpu);
>>>>  	if (fault_status == FSC_PERM && !write_fault) {
>>>> @@ -1009,6 +1053,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  		return -EFAULT;
>>>>  	}
>>>>  
>>>> +
>>>
>>> stray whitespace change?
>> Got it.
>>>
>>>>  	/* 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);
>>>> @@ -1059,12 +1104,35 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  	if (is_error_pfn(pfn))
>>>>  		return -EFAULT;
>>>>  
>>>> -	if (kvm_is_device_pfn(pfn))
>>>> +	if (kvm_is_device_pfn(pfn)) {
>>>>  		mem_type = PAGE_S2_DEVICE;
>>>> +		set_pte_flags = KVM_S2PTE_FLAG_IS_IOMAP;
>>>> +	}
>>>>  
>>>>  	spin_lock(&kvm->mmu_lock);
>>>>  	if (mmu_notifier_retry(kvm, mmu_seq))
>>>>  		goto out_unlock;
>>>> +
>>>> +	/*
>>>> +	 * When logging is enabled general page fault handling changes:
>>>> +	 * -  Writable huge pages are dissolved on a read or write fault.
>>>
>>> why dissolve huge pages on a read fault?
>>
>> What I noticed on write you would dissolve, on read you
>> rebuild THPs, flip back and forth like that, performance
>> & convergence was really bad.
> 
> ah, that makes sense, we should probably indicate that reasoning
> somehow.  In fact, what threw me off was the use of the word "dissolve
> huge pages" which is not really what you're doing on a read fault, there
> you are just never adjusting to huge pages.
> 
> I'm wondering why that would slow things down much though, the only cost
> would be the extra tlb invalidation and replacing the PMD on a
> subsequent write fault, but I trust your numbers nevertheless.
> 
>>>
>>>> +	 * -  pte's are not allowed write permission on a read fault to
>>>> +	 *    writable region so future writes can be marked dirty
>>>
>>> new line
>> ok.
>>>
>>>> +	 * Access to non-writable region is unchanged, and logging of IO
>>>> +	 * regions is not allowed.
>>>> +	 */
>>>> +	if (memslot_is_logging(memslot) && writable) {
>>>> +		set_pte_flags = KVM_S2PTE_FLAG_LOGGING_ACTIVE;
>>>> +		if (hugetlb) {
>>>> +			gfn += pte_index(fault_ipa);
>>>> +			pfn += pte_index(fault_ipa);
>>>> +			hugetlb = false;
>>>> +		}
>>>> +		force_pte = true;
>>>
>>> uh, not this is not what I meant, see my example (untested, partial)
>>> patch in the end of this mail.
>> I put some comments on your patch.
>>>
>>>> +		if (!write_fault)
>>>> +			can_set_pte_rw = false;
>>>> +	}
>>>> +
>>>>  	if (!hugetlb && !force_pte)
>>>>  		hugetlb = transparent_hugepage_adjust(&pfn, &fault_ipa);
>>>>  
>>>> @@ -1082,16 +1150,23 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>  		ret = stage2_set_pmd_huge(kvm, memcache, fault_ipa, &new_pmd);
>>>>  	} else {
>>>>  		pte_t new_pte = pfn_pte(pfn, mem_type);
>>>> -		if (writable) {
>>>> +
>>>> +		/*
>>>> +		 * Don't set write permission, for non-writable region, and
>>>> +		 * for read fault to writable region while logging.
>>>> +		 */
>>>> +		if (writable && can_set_pte_rw) {
>>>>  			kvm_set_s2pte_writable(&new_pte);
>>>>  			kvm_set_pfn_dirty(pfn);
>>>>  		}
>>>>  		coherent_cache_guest_page(vcpu, hva, PAGE_SIZE,
>>>>  					  fault_ipa_uncached);
>>>>  		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte,
>>>> -			pgprot_val(mem_type) == pgprot_val(PAGE_S2_DEVICE));
>>>> +							set_pte_flags);
>>>>  	}
>>>>  
>>>> +	if (write_fault)
>>>> +		mark_page_dirty(kvm, gfn);
>>>>  
>>>>  out_unlock:
>>>>  	spin_unlock(&kvm->mmu_lock);
>>>> @@ -1242,7 +1317,14 @@ 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);
>>>> +	/*
>>>> +	 * We can always call stage2_set_pte with KVM_S2PTE_FLAG_LOGGING_ACTIVE
>>>> +	 * flag clear 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.
>>>> +	 */
>>>> +	stage2_set_pte(kvm, NULL, gpa, pte, 0);
>>>>  }
>>>>  
>>>>  
>>>> @@ -1396,7 +1478,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>>>  	bool writable = !(mem->flags & KVM_MEM_READONLY);
>>>>  	int ret = 0;
>>>>  
>>>> -	if (change != KVM_MR_CREATE && change != KVM_MR_MOVE)
>>>> +	/*
>>>> +	 * Let - enable of dirty page logging through, later check if it's for
>>>> +	 * an IO region and fail.
>>>> +	 */
>>>
>>> I don't understand this comment or find it helpful.
>> Will remove.
>>>
>>>> +	if (change != KVM_MR_CREATE && change != KVM_MR_MOVE &&
>>>> +		change == KVM_MR_FLAGS_ONLY &&
>>>> +		!(memslot->flags & KVM_MEM_LOG_DIRTY_PAGES))
>>>
>>> this looks wrong, because you can now remove all the other checks of
>>> change != and you are not returning early for KVM_MR_DELETE.
>>>
>>> I think you want to add a check simply for 'change != KVM_MR_FLAGS_ONLY'
>>> and then after the 'return 0' check the subconditions for change ==
>>> KVM_MR_FLAGS_ONLY.
>> Yeah, oh boy time to get a new batch of brown bags.
>>
>> I was trying to limit conditional down to add, remap and
>> dirty page flag only in case some other flags get toggled
>> often and waste time walking through VMAs.
>>>
>>>>  		return 0;
>>>>  
>>>>  	/*
>>>> @@ -1447,15 +1535,24 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>>>  			phys_addr_t pa = (vma->vm_pgoff << PAGE_SHIFT) +
>>>>  					 vm_start - vma->vm_start;
>>>>  
>>>> -			ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
>>>> +			if (change != KVM_MR_FLAGS_ONLY)
>>>> +				ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
>>>>  						    vm_end - vm_start,
>>>>  						    writable);
>>>> +			else
>>>> +				/* IO region dirty page logging not allowed */
>>>> +				return -EINVAL;
>>>> +
>>>
>>> this whole thing also looks weird.  I think you just need to add a check
>>> before kvm_phys_addr_ioremap() for flags & KVM_MEM_LOG_DIRTY_PAGES and
>>> return an error in that case (you've identified a user attempting to set
>>> dirty page logging on something that points to device memory, it doesn't
>>> matter at this point through which 'change' it is done).
>>
>> Yes explicitly using KVM_MEM_LOG_DIRTY_PAGES is more clear.
>>
>>>
>>>>  			if (ret)
>>>>  				break;
>>>>  		}
>>>>  		hva = vm_end;
>>>>  	} while (hva < reg_end);
>>>>  
>>>> +	/* Anything after here doesn't apply to memslot flag changes */
>>>> +	if (change == KVM_MR_FLAGS_ONLY)
>>>> +		return ret;
>>>> +
>>>>  	spin_lock(&kvm->mmu_lock);
>>>>  	if (ret)
>>>>  		unmap_stage2_range(kvm, mem->guest_phys_addr, mem->memory_size);
>>>> -- 
>>>
>>>
>>> What I meant last time around concerning user_mem_abort was more
>>> something like this:
>>>
>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>> index 1dc9778..38ea58e 100644
>>> --- a/arch/arm/kvm/mmu.c
>>> +++ b/arch/arm/kvm/mmu.c
>>> @@ -935,7 +935,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>  		return -EFAULT;
>>>  	}
>>>  
>>> -	if (is_vm_hugetlb_page(vma)) {
>>> +	/*
>>> +	 * Writes to pages in a memslot with logging enabled are always logged
>>> +	 * on a singe page-by-page basis.
>>> +	 */
>>> +	if (memslot_is_logging(memslot) && write_fault)
>>> +		force_pte = true;
>>
>> If it's a write you take the pte route and
>> dissolves huge page, if it's a read you reconstruct the
>> THP that seems to yield pretty bad results.
> 
> ok, then remove the ' && write_fault' part of the clause.
Hi Christoffer,
 couple comments/questions.

 setting force_pte here, disables huge pages for
non-writable regions.

> 
>>> +
>>> +	if (is_vm_hugetlb_page(vma) && !force_pte) {
>>>  		hugetlb = true;
>>>  		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>>>  	} else {
>>> @@ -976,6 +983,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>  	if (is_error_pfn(pfn))
>>>  		return -EFAULT;
>>>  
Perhaps here we should check if writable is set
and handle logging, if not do nothing.

>>> +	if (memslot_is_logging(memslot) && !write_fault)
>>> +		writable = false;
>> Ok reusing writable is better.
>>> +
>>>  	if (kvm_is_device_pfn(pfn))
>>>  		mem_type = PAGE_S2_DEVICE;

If we're not setting the IOMAP flag do we have need
this, since we're forfeiting error checking later
in stage2_set_pte()?

Thanks,
  Mario

>>>  
>>> @@ -998,15 +1008,23 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>  					  fault_ipa_uncached);
>>>  		ret = stage2_set_pmd_huge(kvm, memcache, fault_ipa, &new_pmd);
>>>  	} else {
>>> +		unsigned long flags = 0;
>>>  		pte_t new_pte = pfn_pte(pfn, mem_type);
>>> +
>>>  		if (writable) {
>>>  			kvm_set_s2pte_writable(&new_pte);
>>>  			kvm_set_pfn_dirty(pfn);
>>>  		}
>>>  		coherent_cache_guest_page(vcpu, hva, PAGE_SIZE,
>>>  					  fault_ipa_uncached);
>>> -		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte,
>>> -			pgprot_val(mem_type) == pgprot_val(PAGE_S2_DEVICE));
>>> +
>>> +		if (pgprot_val(mem_type) == pgprot_val(PAGE_S2_DEVICE))
>>> +			flags |= KVM_S2PTE_FLAG_IS_IOMAP;
>>> +
>>> +		if (memslot_is_logging(memslot))
>>> +			flags |= KVM_S2_FLAG_LOGGING_ACTIVE;
>> Now that it either IOMAP or LOGGING_ACTIVE do we need to acumulate flags?
>> Although we don't know if device mappings will be handled here.
>>
> 
> so forget all I said about this in the past, I confused the code
> checking for !cache with the iomap flag.
> 
> So, I think you can always safeful assume that stage2_get_pmd() gives you
> something valid back when you have the LOGGING flag set, because you
> always call the function with a valid cache when the LOGGING flag is
> set.  It could be worth adding the following to stage2_set_pte():
> 
> VM_BUG_ON((flags & KVM_S2_FLAG_LOGGING_ACTIVE) && !cache)
> 
> As for this code, the IOMAP flag's only effect is that we return -EFAULT
> if we are seeing an existing PTE for the faulting address.  This would
> no longer be valid if we allow logging dirty device memory pages, so we
> really need to think about if there's any conceivable use case for this?
> 
> It doesn't really make sense to me, so I would suggest that we never
> enable logging for pages that return kvm_is_device_pfn().
> 
> Thanks,
> -Christoffer
>