Re: [PATCH v5 1/7] KVM: arm64: Skip cache maintenance for non-cacheable pKVM mappings

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Jul 17, 2026 at 02:03:11PM +0100, Fuad Tabba wrote:
> From: Bradley Morgan <include@grrlz.net>
> 
> The pKVM flush path walks its own pkvm_mappings list and cleans the
> data cache for every mapping, unlike the generic stage-2 walker it
> shadows, which skips non-cacheable leaves. Cleaning the cacheable
> alias of a non-cacheable mapping is pointless and can corrupt a
> device endpoint. Record whether a mapping is non-cacheable in spare
> bits of nr_pages and skip cache maintenance for it.
> 
> Fixes: e912efed485a ("KVM: arm64: Introduce the EL1 pKVM MMU")
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Bradley Morgan <include@grrlz.net>
> [tabba: use Marc's anonymous bitfield in place of the open-coded mask and helpers]
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

Reviewed-by: Vincent Donnefort <vdonnefort@google.com>

> ---
>  arch/arm64/include/asm/kvm_pkvm.h |  5 ++++-
>  arch/arm64/kvm/pkvm.c             | 15 +++++++++------
>  2 files changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pkvm.h b/arch/arm64/include/asm/kvm_pkvm.h
> index 74fedd9c5ff0..57afb07d6b13 100644
> --- a/arch/arm64/include/asm/kvm_pkvm.h
> +++ b/arch/arm64/include/asm/kvm_pkvm.h
> @@ -195,7 +195,10 @@ struct pkvm_mapping {
>  	struct rb_node node;
>  	u64 gfn;
>  	u64 pfn;
> -	u64 nr_pages;
> +	struct {
> +		u64 nr_pages:48;
> +		u64 nc:1;
> +	};
>  	u64 __subtree_last;	/* Internal member for interval tree */
>  };
>  
> diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
> index 053e4f733e4b..f70c601dcf4c 100644
> --- a/arch/arm64/kvm/pkvm.c
> +++ b/arch/arm64/kvm/pkvm.c
> @@ -369,7 +369,7 @@ static int __pkvm_pgtable_stage2_unshare(struct kvm_pgtable *pgt, u64 start, u64
>  
>  	for_each_mapping_in_range_safe(pgt, start, end, mapping) {
>  		ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_guest, handle, mapping->gfn,
> -					mapping->nr_pages);
> +					(u64)mapping->nr_pages);
>  		if (WARN_ON(ret))
>  			return ret;
>  		pkvm_mapping_remove(mapping, &pgt->pkvm_mappings);
> @@ -473,6 +473,7 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  	mapping->gfn = gfn;
>  	mapping->pfn = pfn;
>  	mapping->nr_pages = size / PAGE_SIZE;
> +	mapping->nc = !!(prot & (KVM_PGTABLE_PROT_DEVICE | KVM_PGTABLE_PROT_NORMAL_NC));
>  	pkvm_mapping_insert(mapping, &pgt->pkvm_mappings);
>  
>  	return ret;
> @@ -503,7 +504,7 @@ int pkvm_pgtable_stage2_wrprotect(struct kvm_pgtable *pgt, u64 addr, u64 size)
>  	lockdep_assert_held(&kvm->mmu_lock);
>  	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
>  		ret = kvm_call_hyp_nvhe(__pkvm_host_wrprotect_guest, handle, mapping->gfn,
> -					mapping->nr_pages);
> +					(u64)mapping->nr_pages);
>  		if (WARN_ON(ret))
>  			break;
>  	}
> @@ -517,9 +518,11 @@ int pkvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
>  	struct pkvm_mapping *mapping;
>  
>  	lockdep_assert_held(&kvm->mmu_lock);
> -	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
> -		__clean_dcache_guest_page(pfn_to_kaddr(mapping->pfn),
> -					  PAGE_SIZE * mapping->nr_pages);
> +	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
> +		if (!mapping->nc)
> +			__clean_dcache_guest_page(pfn_to_kaddr(mapping->pfn),
> +						  PAGE_SIZE * mapping->nr_pages);
> +	}
>  
>  	return 0;
>  }
> @@ -537,7 +540,7 @@ bool pkvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr, u64
>  	lockdep_assert_held(&kvm->mmu_lock);
>  	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
>  		young |= kvm_call_hyp_nvhe(__pkvm_host_test_clear_young_guest, handle, mapping->gfn,
> -					   mapping->nr_pages, mkold);
> +					   (u64)mapping->nr_pages, mkold);
>  
>  	return young;
>  }
> -- 
> 2.39.5
> 

-- 
-- 
Vincent