linux-kernel - Re: [PATCH v2 5/7] KVM: arm64: MTE: Use stage-2 NoTagAccess memory attribute if supported

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Message-ID: <yq5amsfxtrlw.fsf@kernel.org>
Date: Sat, 11 Jan 2025 18:49:55 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
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>, Marc Zyngier <maz@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH v2 5/7] KVM: arm64: MTE: Use stage-2 NoTagAccess memory
 attribute if supported

Catalin Marinas <catalin.marinas@....com> writes:

> On Fri, Jan 10, 2025 at 04:30:21PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> Currently, the kernel won't start a guest if the MTE feature is enabled

...

>> @@ -2152,7 +2162,8 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>  		if (!vma)
>>  			break;
>>  
>> -		if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) {
>> +		if (kvm_has_mte(kvm) &&
>> +		    !kvm_has_mte_perm(kvm) && !kvm_vma_mte_allowed(vma)) {
>>  			ret = -EINVAL;
>>  			break;
>>  		}
>
> I don't think we should change this, or at least not how it's done above
> (Suzuki raised a related issue internally relaxing this for VM_PFNMAP).
>
> For standard memory slots, we want to reject them upfront rather than
> deferring to the fault handler. An example here is file mmap() passed as
> standard RAM to the VM. It's an unnecessary change in behaviour IMHO.
> I'd only relax this for VM_PFNMAP mappings further down in this
> function (and move the VM_PFNMAP check above; see Suzuki's internal
> patch, unless he posted it publicly already).
>

But we want to handle memslots backed by pagecache pages for virtio-shm
here (virtiofs dax use case). With MTE_PERM, we can essentially skip the
kvm_vma_mte_allowed(vma) check because we handle all types in the fault
handler.


-aneesh