From: Andi Kleen Subject: Re: [PATCH v1 4/4] KVM/vmx: enable lbr for the guest Date: Mon, 25 Sep 2017 07:57:20 -0700 Message-ID: <20170925145720.GM4311@tassilo.jf.intel.com> References: <1506314696-4632-1-git-send-email-wei.w.wang@intel.com> <1506314696-4632-5-git-send-email-wei.w.wang@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, mst@redhat.com, rkrcmar@redhat.com, mingo@redhat.com To: Wei Wang Return-path: Content-Disposition: inline In-Reply-To: <1506314696-4632-5-git-send-email-wei.w.wang@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org > +static void auto_switch_lbr_msrs(struct vcpu_vmx *vmx) > +{ > + int i; > + struct perf_lbr_stack lbr_stack; > + > + perf_get_lbr_stack(&lbr_stack); > + > + add_atomic_switch_msr(vmx, MSR_LBR_SELECT, 0, 0); > + add_atomic_switch_msr(vmx, lbr_stack.lbr_tos, 0, 0); > + > + for (i = 0; i < lbr_stack.lbr_nr; i++) { > + add_atomic_switch_msr(vmx, lbr_stack.lbr_from + i, 0, 0); > + add_atomic_switch_msr(vmx, lbr_stack.lbr_to + i, 0, 0); > + if (lbr_stack.lbr_info) > + add_atomic_switch_msr(vmx, lbr_stack.lbr_info + i, 0, > + 0); > + } That will be really expensive and add a lot of overhead to every entry/exit. perf can already context switch the LBRs on task context switch. With that you can just switch LBR_SELECT, which is *much* cheaper because there are far less context switches than exit/entries. It implies that when KVM is running it needs to prevent perf from enabling LBRs in the context of KVM, but that should be straight forward. -Andi