linux-kernel - Re: [PATCH v9 2/6] KVM: arm64: Update the check to detect device memory

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Message-ID: <00e2cdba-0fa3-48da-9596-7a2020bd1858@redhat.com>
Date: Fri, 4 Jul 2025 16:02:11 +0200
From: David Hildenbrand <david@...hat.com>
To: ankita@...dia.com, jgg@...dia.com, maz@...nel.org,
 oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
 yuzenghui@...wei.com, catalin.marinas@....com, will@...nel.org,
 ryan.roberts@....com, shahuang@...hat.com, lpieralisi@...nel.org,
 ddutile@...hat.com, seanjc@...gle.com
Cc: aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
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Subject: Re: [PATCH v9 2/6] KVM: arm64: Update the check to detect device
 memory

On 21.06.25 06:21, ankita@...dia.com wrote:
> From: Ankit Agrawal <ankita@...dia.com>
> 
> Currently, the kvm_is_device_pfn() detects if the memory is kernel
> mapped through pfn_is_map_memory. It thus implies whether KVM can
> use Cache Maintenance Operations (CMOs) on that PFN. It is a bit
> of a misnomer as it does not necessarily detect whether a PFN
> is for a device memory. Moreover, the function is only used at
> one place.
> 
> It would be better to directly call pfn_is_map_memory. Moreover
> we should restrict this call to VM_PFNMAP or VM_MIXEDMAP. Otherwise
> the VMA has struct pages in it and mapped in the kernel VA.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
> ---
>   arch/arm64/kvm/mmu.c | 10 ++++------
>   1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 1601ab9527d4..5fe24f30999d 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -193,11 +193,6 @@ int kvm_arch_flush_remote_tlbs_range(struct kvm *kvm,
>   	return 0;
>   }
>   
> -static bool kvm_is_device_pfn(unsigned long pfn)
> -{
> -	return !pfn_is_map_memory(pfn);
> -}
> -
>   static void *stage2_memcache_zalloc_page(void *arg)
>   {
>   	struct kvm_mmu_memory_cache *mc = arg;
> @@ -1492,6 +1487,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
>   	struct kvm_pgtable *pgt;
>   	struct page *page;
> +	vm_flags_t vm_flags;
>   	enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_HANDLE_FAULT | KVM_PGTABLE_WALK_SHARED;
>   
>   	if (fault_is_perm)
> @@ -1619,6 +1615,8 @@ 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;
>   
> +	vm_flags = vma->vm_flags;
> +
>   	/* Don't use the VMA after the unlock -- it may have vanished */
>   	vma = NULL;
>   
> @@ -1642,7 +1640,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	if (is_error_noslot_pfn(pfn))
>   		return -EFAULT;
>   
> -	if (kvm_is_device_pfn(pfn)) {
> +	if (vm_flags & (VM_PFNMAP | VM_MIXEDMAP) && !pfn_is_map_memory(pfn)) {

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb