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[213.175.37.10]) by smtp.gmail.com with ESMTPSA id e18sm29407471wrw.70.2020.02.04.06.36.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Feb 2020 06:36:54 -0800 (PST) From: Vitaly Kuznetsov To: Wanpeng Li Cc: LKML , kvm , Paolo Bonzini , Sean Christopherson , Wanpeng Li , Jim Mattson , Joerg Roedel , Thadeu Lima de Souza Cascardo Subject: Re: [PATCH] KVM: Pre-allocate 1 cpumask variable per cpu for both pv tlb and pv ipis In-Reply-To: References: <878slio6hp.fsf@vitty.brq.redhat.com> Date: Tue, 04 Feb 2020 15:36:53 +0100 Message-ID: <874kw6o1ve.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wanpeng Li writes: >> >> Honestly, I'd simplify the check in kvm_alloc_cpumask() as >> >> if (!kvm_para_available()) >> return; >> >> and allocated masks for all other cases. > > This will waste the memory if pv tlb and pv ipis are not exposed which > are the only users currently. > My assumption is that the number of cases where we a) expose KVM b) don't expose IPIs and PV-TLB and c) care about 1 cpumask per cpu is relatively low. Ok, let's at least have a function for if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) && !kvm_para_has_hint(KVM_HINTS_REALTIME) && kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) as we now check it twice: in kvm_alloc_cpumask() and kvm_guest_init(), something like pv_tlb_flush_supported(). We can also do pv_ipi_supported() and probably others for consistency. Also, probably not for this patch but it all makes me wonder why there's no per-cpu 'scratch' cpumask for the whole kernel to use. We definitely need it for hypervisor support but I also see arch/x86/kernel/apic/x2apic_cluster.c has similar needs. -- Vitaly