Re: [PATCH v3 2/7] KVM: arm64: Simplify the sanitise_mte_tags() logic

[Steven Price <steven.price@arm.com>]

On 10/08/2022 20:30, Peter Collingbourne wrote:
> From: Catalin Marinas <catalin.marinas@arm.com>
> 
> Currently sanitise_mte_tags() checks if it's an online page before
> attempting to sanitise the tags. Such detection should be done in the
> caller via the VM_MTE_ALLOWED vma flag. Since kvm_set_spte_gfn() does
> not have the vma, leave the page unmapped if not already tagged. Tag
> initialisation will be done on a subsequent access fault in
> user_mem_abort().

Looks correct to me.

Reviewed-by: Steven Price <steven.price@arm.com>

> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Peter Collingbourne <pcc@google.com>
> ---
>  arch/arm64/kvm/mmu.c | 40 +++++++++++++++-------------------------
>  1 file changed, 15 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c9012707f69c..1a3707aeb41f 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1056,23 +1056,14 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>   * - mmap_lock protects between a VM faulting a page in and the VMM performing
>   *   an mprotect() to add VM_MTE
>   */
> -static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> -			     unsigned long size)
> +static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> +			      unsigned long size)
>  {
>  	unsigned long i, nr_pages = size >> PAGE_SHIFT;
> -	struct page *page;
> +	struct page *page = pfn_to_page(pfn);
>  
>  	if (!kvm_has_mte(kvm))
> -		return 0;
> -
> -	/*
> -	 * pfn_to_online_page() is used to reject ZONE_DEVICE pages
> -	 * that may not support tags.
> -	 */
> -	page = pfn_to_online_page(pfn);
> -
> -	if (!page)
> -		return -EFAULT;
> +		return;
>  
>  	for (i = 0; i < nr_pages; i++, page++) {
>  		if (!page_mte_tagged(page)) {
> @@ -1080,8 +1071,6 @@ static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>  			set_page_mte_tagged(page);
>  		}
>  	}
> -
> -	return 0;
>  }
>  
>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> @@ -1092,7 +1081,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	bool write_fault, writable, force_pte = false;
>  	bool exec_fault;
>  	bool device = false;
> -	bool shared;
>  	unsigned long mmu_seq;
>  	struct kvm *kvm = vcpu->kvm;
>  	struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
> @@ -1142,8 +1130,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		vma_shift = get_vma_page_shift(vma, hva);
>  	}
>  
> -	shared = (vma->vm_flags & VM_SHARED);
> -
>  	switch (vma_shift) {
>  #ifndef __PAGETABLE_PMD_FOLDED
>  	case PUD_SHIFT:
> @@ -1264,12 +1250,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  
>  	if (fault_status != FSC_PERM && !device && kvm_has_mte(kvm)) {
>  		/* Check the VMM hasn't introduced a new VM_SHARED VMA */
> -		if (!shared)
> -			ret = sanitise_mte_tags(kvm, pfn, vma_pagesize);
> -		else
> +		if ((vma->vm_flags & VM_MTE_ALLOWED) &&
> +		    !(vma->vm_flags & VM_SHARED)) {
> +			sanitise_mte_tags(kvm, pfn, vma_pagesize);
> +		} else {
>  			ret = -EFAULT;
> -		if (ret)
>  			goto out_unlock;
> +		}
>  	}
>  
>  	if (writable)
> @@ -1491,15 +1478,18 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	kvm_pfn_t pfn = pte_pfn(range->pte);
> -	int ret;
>  
>  	if (!kvm->arch.mmu.pgt)
>  		return false;
>  
>  	WARN_ON(range->end - range->start != 1);
>  
> -	ret = sanitise_mte_tags(kvm, pfn, PAGE_SIZE);
> -	if (ret)
> +	/*
> +	 * If the page isn't tagged, defer to user_mem_abort() for sanitising
> +	 * the MTE tags. The S2 pte should have been unmapped by
> +	 * mmu_notifier_invalidate_range_end().
> +	 */
> +	if (kvm_has_mte(kvm) && !page_mte_tagged(pfn_to_page(pfn)))
>  		return false;
>  
>  	/*

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