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

From: [email-masked] (Mario Smarduch)
Date: Mon, 12 Jan 2015 08:27:03 -0800

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.
> 
>>
>> 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.
> 
>> +	 * -  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.
> +
> +	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;
>  
> +	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;
>  
> @@ -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.

Thanks.
> +
> +		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, flags);
>  	}
>  
>  
> 
> Thanks,
> -Christoffer
>