linux-kernel - Re: [PATCH 3/4] arm64: mte: update code comments

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Message-ID: <87plnktt2q.wl-maz@kernel.org>
Date: Mon, 28 Oct 2024 10:33:49 +0000
From: Marc Zyngier <maz@...nel.org>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>,
	Will Deacon <will@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH 3/4] arm64: mte: update code comments

On Mon, 28 Oct 2024 09:40:13 +0000,
"Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org> wrote:
> 
> commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
> initialisation") updated the locking such the kernel now allows
> VM_SHARED mapping with MTE. Update the code comment to reflect this.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> ---
>  arch/arm64/kvm/mmu.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)

This is a KVM patch. Please make sure you write the subject
accordingly, matching the existing conventions (in this case, this
should read something like: "KVM: arm64: MTE: Update...").

> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index a509b63bd4dd..b5824e93cee0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1390,11 +1390,8 @@ 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
> + * The race in the test/set of the PG_mte_tagged flag is handled by
> + * using PG_mte_lock and PG_mte_tagged together.

How? This comment is pretty content-free. TO be useful, you should
elaborate on *how* these two are used together.

>   */
>  static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>  			      unsigned long size)
> @@ -1646,7 +1643,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	}
>  
>  	if (!fault_is_perm && !device && kvm_has_mte(kvm)) {
> -		/* Check the VMM hasn't introduced a new disallowed VMA */
> +		/*
> +		 *  not a permission fault implies a translation fault which
> +		 *  means mapping the page for the first time

How about an Access fault due to page ageing?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.