linux-kernel - Re: [PATCH v7 2/5] KVM: arm64: Block cacheable PFNMAP mapping

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Message-ID: <aFLfT_c4Fcxt8euY@arm.com>
Date: Wed, 18 Jun 2025 16:46:23 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: ankita@...dia.com
Cc: jgg@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
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Subject: Re: [PATCH v7 2/5] KVM: arm64: Block cacheable PFNMAP mapping

On Wed, Jun 18, 2025 at 06:55:38AM +0000, ankita@...dia.com wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 3d77a278fc4f..d6e0d5f46b45 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1470,6 +1470,22 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
>  	return vma->vm_flags & VM_MTE_ALLOWED;
>  }
>  
> +/*
> + * Determine the memory region cacheability from VMA's pgprot. This
> + * is used to set the stage 2 PTEs.
> + */
> +static bool kvm_vma_is_cacheable(struct vm_area_struct *vma)
> +{
> +	switch (FIELD_GET(PTE_ATTRINDX_MASK, pgprot_val(vma->vm_page_prot))) {
> +	case MT_NORMAL_NC:
> +	case MT_DEVICE_nGnRnE:
> +	case MT_DEVICE_nGnRE:
> +		return false;
> +	default:
> +		return true;
> +	}
> +}
> +
>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  			  struct kvm_s2_trans *nested,
>  			  struct kvm_memory_slot *memslot, unsigned long hva,
> @@ -1477,7 +1493,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  {
>  	int ret = 0;
>  	bool write_fault, writable, force_pte = false;
> -	bool exec_fault, mte_allowed;
> +	bool exec_fault, mte_allowed, is_vma_cacheable = false;

Nit: do we need to initialise is_vma_cacheable here? It did not seem
used until the kvm_vma_is_cacheable() call. Anyway, it's harmless.

>  	bool disable_cmo = false, vfio_allow_any_uc = false;
>  	unsigned long mmu_seq;
>  	phys_addr_t ipa = fault_ipa;
> @@ -1619,6 +1635,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  
>  	vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
>  
> +	is_vma_cacheable = kvm_vma_is_cacheable(vma);
> +
>  	/* Don't use the VMA after the unlock -- it may have vanished */
>  	vma = NULL;
>  
> @@ -1643,6 +1661,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		return -EFAULT;
>  
>  	if (!kvm_can_use_cmo_pfn(pfn)) {
> +		if (is_vma_cacheable)
> +			return -EINVAL;
> +
>  		/*
>  		 * If the page was identified as device early by looking at
>  		 * the VMA flags, vma_pagesize is already representing the

This block also sets 'disable_cmo' (originally 'device') to true.

> @@ -1726,6 +1747,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		prot |= KVM_PGTABLE_PROT_X;
>  
>  	if (disable_cmo) {
> +		if (is_vma_cacheable) {
> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}

so, is there anything else changing 'disable_cmo' up to this point? If
not, I'd drop the second is_vma_cacheable check.

-- 
Catalin