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

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Message-ID: <yq5attcwcs2l.fsf@kernel.org>
Date: Mon, 28 Oct 2024 18:17:30 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Marc Zyngier <maz@...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

Hi Marc,

Thanks for reviewing the changes.

Marc Zyngier <maz@...nel.org> writes:

> 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...").
>

Will 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.
>

I will add more details described in commit d77e59a8fccde7fb5dd8c57594ed147b4291c970
Should i quote the commit there in the comment? 

>
>>   */
>>  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?
>

IIUC access fault is already handled by the caller kvm_handle_guest_abort?
I can add that as part of the updated comments?

-aneesh