linux-kernel - Re: [PATCH v9 1/8] KVM: arm/arm64: Share common code in user_mem

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Date:   Mon, 10 Dec 2018 09:56:16 +0100
From:   Christoffer Dall <christoffer.dall@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        punitagrawal@...il.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v9 1/8] KVM: arm/arm64: Share common code in
 user_mem_abort()

On Mon, Dec 03, 2018 at 01:37:37PM +0000, Suzuki K Poulose wrote:
> 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@....com>
> >>Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> >>Cc: Christoffer Dall <christoffer.dall@....com>
> >>Cc: Marc Zyngier <marc.zyngier@....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.
> 

I'm ok with mapsize.  I see the vma_pagesize name coming from the fact
that this is initially set to the return value from vma_kernel_pagesize.

I have not problems with either vma_pagesize or mapsize.

> >
> >>  	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.
> 
> 

Agreed, and let's not try additionally change the logic and flow with
this patch.

> >
> >>+
> >>  		/*
> >>  		 * 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().
> 

Agreed.

> >>+		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.
> 

That's fine.

> >
> >>+	}
> >>+
> >>+	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.
> 
Agreed, I don't understand the comment raised by Anshuman here.


Thanks,

    Christoffer