Subject: [RESEND PATCH v15 07/11] KVM: arm: page logging 2nd stage fault handling

From: [email-masked] (Mario Smarduch)
Date: Wed, 07 Jan 2015 17:43:18 -0800

Hi Christoffer,
  before going through your comments, I discovered that
in 3.18.0-rc2 - a generic __get_user_pages_fast()
was implemented, now ARM picks this up. This causes
gfn_to_pfn_prot() to return meaningful 'writable'
value for a read fault, provided the region is writable.

Prior to that the weak version returned 0 and 'writable'
had no optimization effect to set pte/pmd - RW on
a read fault.

As a consequence dirty logging broke in 3.18, I was seeing
weird but very intermittent issues. I just put in the
additional few lines to fix it, prevent pte RW (only R) on
read faults  while  logging writable region.

On 01/07/2015 04:38 AM, Christoffer Dall wrote:
> On Wed, Dec 17, 2014 at 06:07:29PM -0800, Mario Smarduch wrote:
>> This patch is a followup to v15 patch series, with following changes:
>> - When clearing/dissolving a huge, PMD mark huge page range dirty, since
>>   the state of whole range is unknown. After the huge page is dissolved 
>>   dirty page logging is at page granularity.
> 
> What is the sequence of events where you could have dirtied another page
> within the PMD range after the user initially requested dirty page
> logging?

No there is none. My issue was the start point for tracking dirty pages
and that would be second call to dirty log read. Not first
call after initial write protect where any page in range can
be assumed dirty. I'll remove this, not sure if there would be any
use case to call dirty log only once.

> 
>> - Correct comment due to misinterpreted test results
>>
>> Retested, everything appears to work fine. 
> 
> you should resend this with the proper commit message, and changelogs
> should go beneath the '---' separator.

Yes will do.
> 
>>   
>>
>> Signed-off-by: Mario Smarduch <[email-masked]>
>> ---
>>  arch/arm/kvm/mmu.c |   86 +++++++++++++++++++++++++++++++++++++++++++++++-----
>>  1 file changed, 78 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> index 73d506f..7e83a16 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 kvm_get_logging_state(struct kvm_memory_slot *memslot)
> 
> nit: if you respin I think this would be slightly more clear if it was
> named something like memslot_is_logging() - I have a vague feeling I was
> the one who suggested this name in the past but now it annoyes me
> slightly.

Yes, more intuitive.

> 
>> +{
>> +#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 +71,37 @@ 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.
>> + */
>> +void stage2_dissolve_pmd(struct kvm *kvm, phys_addr_t addr, pmd_t *pmd)
> 
> this can be a static
> 
Missed it.
>> +{
>> +	gfn_t gfn;
>> +	int i;
>> +
>> +	if (kvm_pmd_huge(*pmd)) {
> 
> Can you invert this, so you return early if it's not a
> kvm_pmd_huge(*pmd) ?

will do.
> 
>> +
>> +		pmd_clear(pmd);
>> +		kvm_tlb_flush_vmid_ipa(kvm, addr);
>> +		put_page(virt_to_page(pmd));
>> +
>> +		gfn = (addr & PMD_MASK) >> PAGE_SHIFT;
>> +
>> +		/*
>> +		 * The write is to a huge page, mark the whole page dirty
>> +		 * including this gfn.
>> +		 */
> 
> we need the explanation I'm asking for in the commit message as part of
> the comment here. Currently the comment explains what the code is quite
> obviously doing, but not *why*....

That was what I mentioned above, it will be gone.
> 
>> +		for (i = 0; i < PTRS_PER_PMD; i++)
>> +			mark_page_dirty(kvm, gfn + i);
>> +	}
>> +}
>> +
>>  static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
>>  				  int min, int max)
>>  {
>> @@ -703,10 +746,13 @@ 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;
>> +	unsigned long iomap = flags & KVM_S2PTE_FLAG_IS_IOMAP;
>> +	unsigned long logging_active = flags & KVM_S2PTE_FLAG_LOGGING_ACTIVE;
> 
> why not declare these as bool?
Yes.
> 
>>  
>>  	/* Create stage-2 page table mapping - Levels 0 and 1 */
>>  	pmd = stage2_get_pmd(kvm, cache, addr);
>> @@ -718,6 +764,13 @@ 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 +827,8 @@ 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 +1056,7 @@ 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;
>> +	unsigned long logging_active = 0;
> 
> can you change this to a bool and set the flag explicitly once you've
> declared flags further down?  I think that's more clear.
ok.
> 
>>  
>>  	write_fault = kvm_is_write_fault(vcpu);
>>  	if (fault_status == FSC_PERM && !write_fault) {
>> @@ -1009,6 +1064,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  		return -EFAULT;
>>  	}
>>  
>> +	if (kvm_get_logging_state(memslot) && write_fault)
>> +		logging_active = KVM_S2PTE_FLAG_LOGGING_ACTIVE;
>> +
Yes I noticed non writable regions were dissolved, but this
was not the way to go about it. Right now after call is made
to gfn_to_pfn_prot() the snippet is executed to do
nothing for non-writable regions.

if (kvm_get_logging_state(memslot) && writable) {
	logging_active = KVM_S2PTE_FLAG_LOGGING_ACTIVE;
	if (!write_fault)
		 can_set_pte_rw = false;
        gfn = fault_ipa >> PAGE_SHIFT;
        force_pte = true;
}

if (!hugetlb && !force_pte)
 ...

> 
> so if the guest is faulting on a read of a huge page then we're going to
> map it as a huge page, but not if it's faulting on a write.  Why
> exactly?  A slight optimization?  Perhaps it's worth a comment.
> 
>>  	/* 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);
>> @@ -1018,7 +1076,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  		return -EFAULT;
>>  	}
>>  
>> -	if (is_vm_hugetlb_page(vma)) {
>> +	if (is_vm_hugetlb_page(vma) && !logging_active) {

These references to logging_active in conditional checks
are gone.

> 
> So I think this whole thing could look nicer if you set force_pte = true
> together with setting logging_active above, and then change this check
> to check && !force_pte here and get rid of the extra check of
> !logging_active for the THP check below.
> 
> Sorry to be a bit pedantic, but this code is really critical.
> 
>>  		hugetlb = true;
>>  		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>>  	} else {
>> @@ -1065,7 +1123,7 @@ 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);
>>  
>>  	fault_ipa_uncached = memslot->flags & KVM_MEMSLOT_INCOHERENT;
>> @@ -1082,17 +1140,22 @@ 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);
>> +		unsigned long flags = logging_active;
>> +
>> +		if (pgprot_val(mem_type) == pgprot_val(PAGE_S2_DEVICE))
>> +			flags |= KVM_S2PTE_FLAG_IS_IOMAP;
>> +
>>  		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));
>> +		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, flags);
>>  	}
>>  
>> -
>> +	if (write_fault)
>> +		mark_page_dirty(kvm, gfn);
>>  out_unlock:
>>  	spin_unlock(&kvm->mmu_lock);
>>  	kvm_release_pfn_clean(pfn);
>> @@ -1242,7 +1305,14 @@ 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 set because MMU notifiers will have unmapped a huge PMD before
> 
>                 ^^^ surely you mean 'clear', right?
Yes of course, the value is 0 we can only deal with pages here.
> 
>> +	 * 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);
>>  }
>>  
>>  
>> -- 
>> 1.7.9.5
>>
> 
> Thanks,
> -Christoffer
>