Re: [PATCH v9 1/8] KVM: arm/arm64: Share common code in user_mem_abort()

From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	kvmarm@lists.cs.columbia.edu
Cc: marc.zyngier@arm.com, will.deacon@arm.com,
	linux-kernel@vger.kernel.org,
	Christoffer Dall <christoffer.dall@arm.com>,
	punitagrawal@gmail.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v9 1/8] KVM: arm/arm64: Share common code in user_mem_abort()
Date: Mon, 3 Dec 2018 13:37:37 +0000	[thread overview]
Message-ID: <8fd34e5f-7d75-4de2-3fee-d6d70805685c@arm.com> (raw)
In-Reply-To: <ad4b316a-054f-24d9-d32d-8fbe7972b5ad@arm.com>

Hi Anshuman,

On 03/12/2018 12:11, Anshuman Khandual wrote:
> 
> 
> On 10/31/2018 11:27 PM, Punit Agrawal wrote:
>> The code for operations such as marking the pfn as dirty, and
>> dcache/icache maintenance during stage 2 fault handling is duplicated
>> between normal pages and PMD hugepages.
>>
>> Instead of creating another copy of the operations when we introduce
>> PUD hugepages, let's share them across the different pagesizes.
>>
>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>   virt/kvm/arm/mmu.c | 49 ++++++++++++++++++++++++++++------------------
>>   1 file changed, 30 insertions(+), 19 deletions(-)
>>
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 5eca48bdb1a6..59595207c5e1 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1475,7 +1475,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   			  unsigned long fault_status)
>>   {
>>   	int ret;
>> -	bool write_fault, exec_fault, writable, hugetlb = false, force_pte = false;
>> +	bool write_fault, exec_fault, writable, force_pte = false;
>>   	unsigned long mmu_seq;
>>   	gfn_t gfn = fault_ipa >> PAGE_SHIFT;
>>   	struct kvm *kvm = vcpu->kvm;
>> @@ -1484,7 +1484,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	kvm_pfn_t pfn;
>>   	pgprot_t mem_type = PAGE_S2;
>>   	bool logging_active = memslot_is_logging(memslot);
>> -	unsigned long flags = 0;
>> +	unsigned long vma_pagesize, flags = 0;
> 
> A small nit s/vma_pagesize/pagesize. Why call it VMA ? Its implicit.

May be we could call it mapsize. pagesize is confusing.

> 
>>   
>>   	write_fault = kvm_is_write_fault(vcpu);
>>   	exec_fault = kvm_vcpu_trap_is_iabt(vcpu);
>> @@ -1504,10 +1504,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   		return -EFAULT;
>>   	}
>>   
>> -	if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
>> -		hugetlb = true;
>> +	vma_pagesize = vma_kernel_pagesize(vma);
>> +	if (vma_pagesize == PMD_SIZE && !logging_active) {
>>   		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>>   	} else {
>> +		/*
>> +		 * Fallback to PTE if it's not one of the Stage 2
>> +		 * supported hugepage sizes
>> +		 */
>> +		vma_pagesize = PAGE_SIZE;
> 
> This seems redundant and should be dropped. vma_kernel_pagesize() here either
> calls hugetlb_vm_op_pagesize (via hugetlb_vm_ops->pagesize) or simply returns
> PAGE_SIZE. The vm_ops path is taken if the QEMU VMA covering any given HVA is
> backed either by HugeTLB pages or simply normal pages. vma_pagesize would
> either has a value of PMD_SIZE (HugeTLB hstate based) or PAGE_SIZE. Hence if
> its not PMD_SIZE it must be PAGE_SIZE which should not be assigned again.

We may want to force using the PTE mappings when logging_active (e.g, migration
?) to prevent keep tracking of huge pages. So the check is still valid.

> 
>> +
>>   		/*
>>   		 * Pages belonging to memslots that don't have the same
>>   		 * alignment for userspace and IPA cannot be mapped using
>> @@ -1573,23 +1579,33 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	if (mmu_notifier_retry(kvm, mmu_seq))
>>   		goto out_unlock;
>>   
>> -	if (!hugetlb && !force_pte)
>> -		hugetlb = transparent_hugepage_adjust(&pfn, &fault_ipa);
>> +	if (vma_pagesize == PAGE_SIZE && !force_pte) {
>> +		/*
>> +		 * Only PMD_SIZE transparent hugepages(THP) are
>> +		 * currently supported. This code will need to be
>> +		 * updated to support other THP sizes.
>> +		 */
> 
> This comment belongs to transparent_hugepage_adjust() but not here.

I think this is relevant here than in thp_adjust, unless we rename
the function below to something generic, handle_hugepage_adjust().

>> +		if (transparent_hugepage_adjust(&pfn, &fault_ipa))
>> +			vma_pagesize = PMD_SIZE;
> 
> IIUC transparent_hugepage_adjust() is only getting called here. Instead of
> returning 'true' when it is able to detect a huge page backing and doing
> an adjustment there after, it should rather return THP size (PMD_SIZE) to
> accommodate probable multi size THP support in future .

That makes sense.

> 
>> +	}
>> +
>> +	if (writable)
>> +		kvm_set_pfn_dirty(pfn);
>>   
>> -	if (hugetlb) {
>> +	if (fault_status != FSC_PERM)
>> +		clean_dcache_guest_page(pfn, vma_pagesize);
>> +
>> +	if (exec_fault)
>> +		invalidate_icache_guest_page(pfn, vma_pagesize);
>> +
>> +	if (vma_pagesize == PMD_SIZE) {
>>   		pmd_t new_pmd = pfn_pmd(pfn, mem_type);
>>   		new_pmd = pmd_mkhuge(new_pmd);
>> -		if (writable) {
>> +		if (writable)
>>   			new_pmd = kvm_s2pmd_mkwrite(new_pmd);
>> -			kvm_set_pfn_dirty(pfn);
>> -		}
>> -
>> -		if (fault_status != FSC_PERM)
>> -			clean_dcache_guest_page(pfn, PMD_SIZE);
>>   
>>   		if (exec_fault) {
>>   			new_pmd = kvm_s2pmd_mkexec(new_pmd);
>> -			invalidate_icache_guest_page(pfn, PMD_SIZE);
>>   		} else if (fault_status == FSC_PERM) {
>>   			/* Preserve execute if XN was already cleared */
>>   			if (stage2_is_exec(kvm, fault_ipa))
>> @@ -1602,16 +1618,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   
>>   		if (writable) {
>>   			new_pte = kvm_s2pte_mkwrite(new_pte);
>> -			kvm_set_pfn_dirty(pfn);
>>   			mark_page_dirty(kvm, gfn);
>>   		}
>>   
>> -		if (fault_status != FSC_PERM)
>> -			clean_dcache_guest_page(pfn, PAGE_SIZE);
>> -
>>   		if (exec_fault) {
>>   			new_pte = kvm_s2pte_mkexec(new_pte);
>> -			invalidate_icache_guest_page(pfn, PAGE_SIZE);
>>   		} else if (fault_status == FSC_PERM) {
>>   			/* Preserve execute if XN was already cleared */
>>   			if (stage2_is_exec(kvm, fault_ipa))
>>
> 
> kvm_set_pfn_dirty, clean_dcache_guest_page, invalidate_icache_guest_page
> can all be safely moved before setting the page table entries either as
> PMD or PTE.

I think this is what we do currently. So I assume this is fine.

Cheers
Suzuki