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Wed, 28 Jul 2021 14:51:13 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m8kto-0019Ss-Dk for linux-arm-kernel@lists.infradead.org; Wed, 28 Jul 2021 14:51:10 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 86FDB1042; Wed, 28 Jul 2021 07:51:03 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8FDC63F70D; Wed, 28 Jul 2021 07:51:01 -0700 (PDT) Subject: Re: [PATCH 01/16] KVM: arm64: Generalise VM features into a set of flags To: Marc Zyngier , Will Deacon Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, qperret@google.com, dbrazdil@google.com, Srivatsa Vaddagiri , Shanker R Donthineni , James Morse , Suzuki K Poulose , Alexandru Elisei , kernel-team@android.com References: <20210715163159.1480168-1-maz@kernel.org> <20210715163159.1480168-2-maz@kernel.org> <20210727181026.GA19173@willie-the-truck> <875ywuepxv.wl-maz@kernel.org> From: Steven Price Message-ID: <716fffdb-580a-bc70-478a-a54912a77c82@arm.com> Date: Wed, 28 Jul 2021 15:51:00 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <875ywuepxv.wl-maz@kernel.org> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210728_075108_606601_22A0A9FA X-CRM114-Status: GOOD ( 22.64 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 28/07/2021 10:41, Marc Zyngier wrote: > On Tue, 27 Jul 2021 19:10:27 +0100, > Will Deacon wrote: >> >> On Thu, Jul 15, 2021 at 05:31:44PM +0100, Marc Zyngier wrote: >>> We currently deal with a set of booleans for VM features, >>> while they could be better represented as set of flags >>> contained in an unsigned long, similarily to what we are >>> doing on the CPU side. >>> >>> Signed-off-by: Marc Zyngier >>> --- >>> arch/arm64/include/asm/kvm_host.h | 12 +++++++----- >>> arch/arm64/kvm/arm.c | 5 +++-- >>> arch/arm64/kvm/mmio.c | 3 ++- >>> 3 files changed, 12 insertions(+), 8 deletions(-) >>> >>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >>> index 41911585ae0c..4add6c27251f 100644 >>> --- a/arch/arm64/include/asm/kvm_host.h >>> +++ b/arch/arm64/include/asm/kvm_host.h >>> @@ -122,7 +122,10 @@ struct kvm_arch { >>> * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is >>> * supported. >>> */ >>> - bool return_nisv_io_abort_to_user; >>> +#define KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER 0 >>> + /* Memory Tagging Extension enabled for the guest */ >>> +#define KVM_ARCH_FLAG_MTE_ENABLED 1 >>> + unsigned long flags; >> >> One downside of packing all these together is that updating 'flags' now >> requires an atomic rmw sequence (i.e. set_bit()). Then again, that's >> probably for the best anyway given that kvm_vm_ioctl_enable_cap() looks >> like it doesn't hold any locks. > > That, and these operations are supposed to be extremely rare anyway. > >> >>> /* >>> * VM-wide PMU filter, implemented as a bitmap and big enough for >>> @@ -133,9 +136,6 @@ struct kvm_arch { >>> >>> u8 pfr0_csv2; >>> u8 pfr0_csv3; >>> - >>> - /* Memory Tagging Extension enabled for the guest */ >>> - bool mte_enabled; >>> }; >>> >>> struct kvm_vcpu_fault_info { >>> @@ -777,7 +777,9 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu); >>> #define kvm_arm_vcpu_sve_finalized(vcpu) \ >>> ((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED) >>> >>> -#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled) >>> +#define kvm_has_mte(kvm) \ >>> + (system_supports_mte() && \ >>> + test_bit(KVM_ARCH_FLAG_MTE_ENABLED, &(kvm)->arch.flags)) >> >> Not an issue with this patch, but I just noticed that the >> system_supports_mte() check is redundant here as we only allow the flag to >> be set if that's already the case. > > It allows us to save a memory access if system_supports_mte() is false > (it is eventually implemented as a static key). On the other hand, > there is so much inlining due to it being a non-final cap that we > probably lose on that too... My original logic was that system_supports_mte() checks IS_ENABLED(CONFIG_ARM64_MTE) - so this enables the code guarded with kvm_has_mte() to be compiled out if CONFIG_ARM64_MTE is disabled. Indeed it turns at we currently rely on this (with CONFIG_ARM64_MTE disabled): aarch64-linux-gnu-ld: arch/arm64/kvm/mmu.o: in function `sanitise_mte_tags': /home/stepri01/work/linux/arch/arm64/kvm/mmu.c:887: undefined reference to `mte_clear_page_tags' aarch64-linux-gnu-ld: arch/arm64/kvm/guest.o: in function `kvm_vm_ioctl_mte_copy_tags': /home/stepri01/work/linux/arch/arm64/kvm/guest.c:1066: undefined reference to `mte_copy_tags_to_user' aarch64-linux-gnu-ld: /home/stepri01/work/linux/arch/arm64/kvm/guest.c:1074: undefined reference to `mte_copy_tags_from_user' Obviously we could pull just the IS_ENABLED() into kvm_has_mte() instead. Steve _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel