Re: [PATCH v3 6/7] KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled

From: Steven Price <steven.price@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Peter Collingbourne <pcc@google.com>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, Cornelia Huck <cohuck@redhat.com>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Evgenii Stepanov <eugenis@google.com>,
	kvm@vger.kernel.org,
	Vincenzo Frascino <vincenzo.frascino@arm.com>
Subject: Re: [PATCH v3 6/7] KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled
Date: Fri, 2 Sep 2022 15:47:40 +0100	[thread overview]
Message-ID: <5413e00f-251f-9d48-9cbb-07742feec87f@arm.com> (raw)
In-Reply-To: <YxII905jjQz0FH4D@arm.com>

On 02/09/2022 14:45, Catalin Marinas wrote:
> On Wed, Aug 10, 2022 at 12:30:32PM -0700, Peter Collingbourne wrote:
>> Certain VMMs such as crosvm have features (e.g. sandboxing) that depend
>> on being able to map guest memory as MAP_SHARED. The current restriction
>> on sharing MAP_SHARED pages with the guest is preventing the use of
>> those features with MTE. Now that the races between tasks concurrently
>> clearing tags on the same page have been fixed, remove this restriction.
>>
>> Signed-off-by: Peter Collingbourne <pcc@google.com>
>> ---
>>  arch/arm64/kvm/mmu.c | 8 --------
>>  1 file changed, 8 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index d54be80e31dd..fc65dc20655d 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1075,14 +1075,6 @@ static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>>  
>>  static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
>>  {
>> -	/*
>> -	 * VM_SHARED mappings are not allowed with MTE to avoid races
>> -	 * when updating the PG_mte_tagged page flag, see
>> -	 * sanitise_mte_tags for more details.
>> -	 */
>> -	if (vma->vm_flags & VM_SHARED)
>> -		return false;
> 
> I think this is fine with the locking in place (BTW, it may be worth
> mentioning in the commit message that it's a relaxation of the ABI). I'd
> like Steven to have a look as well when he gets the time, in case we
> missed anything on the KVM+MTE side.
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Looks fine to me, and thanks for doing the work: I was never very
pleased with the !VM_SHARED restriction, but I couldn't figure a good
way of getting the locking to work.

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

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