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

From: [email-masked] (Mario Smarduch)
Date: Tue, 13 Jan 2015 09:42:34 -0800

On 01/12/2015 11:43 AM, Christoffer Dall wrote:
> On Mon, Jan 12, 2015 at 11:04:45AM -0800, Mario Smarduch wrote:
> 
> [...]
> 
>>>>>> @@ -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.
>>
>> If I understand correctly -
>> you do few writes, dissolve a huge page insert pte TLB entries,
>> then a read page fault installs a pmd clears the TLB cache
>> for that range, and it repeats over. Appears like you
>> need to constantly re-fault pte TLBs on writes to huge
>> page range.
> 
> that makes good sense, thanks for the explanation.
> 
> [...]
> 
>>>>>  	} 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)
>>
>> I see ok, thanks for clearing that up.
>>
>>>
>>> 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
>> Sorry, do you mean allow or disallow?
> 
> if we (by these patches) allow logging dirty pages for device memory,
> then we...
> 
>>
>>> really need to think about if there's any conceivable use case for this?

No I can't think of any use case to log Device address space.

So I could move forward - drop the IOMAP flag, and add the
VM_BUG_ON to stage2_set_pte().

Thanks.

>>>
>>> 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
>>>
>>