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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id n14sm60042pgd.48.2021.09.10.16.55.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Sep 2021 16:55:29 -0700 (PDT) Date: Fri, 10 Sep 2021 23:55:25 +0000 From: Sean Christopherson To: Zeng Guang Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, Dave Hansen , Tony Luck , Kan Liang , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Kim Phillips , Jarkko Sakkinen , Jethro Beekman , Kai Huang , x86@kernel.org, linux-kernel@vger.kernel.org, Robert Hu , Gao Chao Subject: Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization Message-ID: References: <20210809032925.3548-1-guang.zeng@intel.com> <20210809032925.3548-7-guang.zeng@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 10, 2021, Sean Christopherson wrote: > On Mon, Aug 09, 2021, Zeng Guang wrote: > > + if (!pages) > > + return -ENOMEM; > > + > > + to_kvm_vmx(kvm)->pid_table = (void *)page_address(pages); > > + to_kvm_vmx(kvm)->pid_last_index = KVM_MAX_VCPU_ID; > > I don't see the point of pid_last_index if we're hardcoding it to KVM_MAX_VCPU_ID. > If I understand the ucode pseudocode, there's no performance hit in the happy > case, i.e. it only guards against out-of-bounds accesses. > > And I wonder if we want to fail the build if this grows beyond an order-1 > allocation, e.g. > > BUILD_BUG_ON(PID_TABLE_ORDER > 1); > > Allocating two pages per VM isn't terrible, but 4+ starts to get painful when > considering the fact that most VMs aren't going to need more than one page. For > now I agree the simplicity of not dynamically growing the table is worth burning > a page. Ugh, Paolo has queued a series which bumps KVM_MAX_VCPU_ID to 4096[*]. That makes this an order-3 allocation, which is quite painful. One thought would be to let userspace declare the max vCPU it wants to create, not sure if that would work for xAPIC though. [*] https://lkml.kernel.org/r/1111efc8-b32f-bd50-2c0f-4c6f506b544b@redhat.com