linux-kernel - Re: [PATCH] KVM: arm64: Drop mte_allowed check during memslot creation

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Message-ID: <a8ceea52-e34a-4edb-bd54-ca30ec030432@arm.com>
Date: Mon, 24 Feb 2025 10:32:01 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 kvmarm@...ts.linux.dev
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Joey Gouly <joey.gouly@....com>, Zenghui Yu <yuzenghui@...wei.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Steven Price <steven.price@....com>
Subject: Re: [PATCH] KVM: arm64: Drop mte_allowed check during memslot
 creation

On 24/02/2025 09:39, Aneesh Kumar K.V (Arm) wrote:
> Before commit d89585fbb308 ("KVM: arm64: unify the tests for VMAs in
> memslots when MTE is enabled"), kvm_arch_prepare_memory_region() only
> rejected a memory slot if VM_SHARED was set. This commit unified the
> checking with user_mem_abort(), with slots being rejected if either
> VM_MTE_ALLOWED is not set or VM_SHARED set. A subsequent commit
> c911f0d46879 ("KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE
> enabled") dropped the VM_SHARED check, so we ended up with memory slots
> being rejected if VM_MTE_ALLOWED is not set. This wasn't the case before
> the commit d89585fbb308. The rejection of the memory slot with VM_SHARED
> set was done to avoid a race condition with the test/set of the
> PG_mte_tagged flag. Before Commit d77e59a8fccd ("arm64: mte: Lock a page
> for MTE tag initialization") the kernel avoided allowing MTE with shared
> pages, thereby preventing two tasks sharing a page from setting up the
> PG_mte_tagged flag racily.
> 
> Commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
> initialization") further updated the locking so that the kernel
> allows VM_SHARED mapping with MTE. With this commit, we can enable
> memslot creation with VM_SHARED VMA mapping.
> 
> This patch results in a minor tweak to the ABI. We now allow creating
> memslots that don't have the VM_MTE_ALLOWED flag set. If the guest uses
> such a memslot with Allocation Tags, the kernel will generate -EFAULT.
> ie, instead of failing early, we now fail later during KVM_RUN.
> 
> This change is needed because, without it, users are not able to use MTE
> with VFIO passthrough (currently the mapping is either Device or
> NonCacheable for which tag access check is not applied.), as shown
> below (kvmtool VMM).
> 
> [  617.921030] vfio-pci 0000:01:00.0: resetting
> [  618.024719] vfio-pci 0000:01:00.0: reset done
>    Error: 0000:01:00.0: failed to register region with KVM
>    Warning: [0abc:aced] Error activating emulation for BAR 0
>    Error: 0000:01:00.0: failed to configure regions
>    Warning: Failed init: vfio__init
> 
>    Fatal: Initialisation failed
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>

Tested-by: Suzuki K Poulose <suzuki.poulose@....com>