[PATCH] KVM: arm64: Update stale comment for sanitise_mte_tags()

[Steven Price]

On 15/09/2025 16:52, Alexandru Elisei wrote:
> Commit c911f0d46879 ("KVM: arm64: permit all VM_MTE_ALLOWED mappings
> with MTE enabled") allowed VM_SHARED VMAs in a VM with MTE enabled, so
> remove the comment to the contrary.
> 
> Commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag initialisation")
> removed the race that can lead to tags being zeroed more than once when
> multiple threads attempt initialisation at the same time, so remove the
> comment about mmap_lock too. Note that sanitise_mte_tags() was never called
> with the mmap_lock held from user_mem_abort() and the race was prevented by
> kvm->mmu_lock.
> 
> However, the function still requires to have the kvm->mmu_lock held to
> ensure that the memory remains mapped in the userspace process while the
> tags are zeroed. Document this in a comment.
> 
> CC: Peter Collingbourne <pcc at google.com>
> CC: Catalin Marinas <catalin.marinas at arm.com>
> CC: Steven Price <steven.price at arm.com>
> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>

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

Thanks,
Steve

> ---
>  arch/arm64/kvm/mmu.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 86f3d80daf37..33f4c923be70 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1459,11 +1459,8 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>   * able to see the page's tags and therefore they must be initialised first. If
>   * PG_mte_tagged is set, tags have already been initialised.
>   *
> - * The race in the test/set of the PG_mte_tagged flag is handled by:
> - * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
> - *   racing to santise the same page
> - * - mmap_lock protects between a VM faulting a page in and the VMM performing
> - *   an mprotect() to add VM_MTE
> + * Must be called with kvm->mmu_lock held to ensure the memory remains mapped
> + * while the tags are zeroed.
>   */
>  static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>  			      unsigned long size)
> 
> base-commit: f83ec76bf285bea5727f478a68b894f5543ca76e