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    SubjectRe: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory
    2017-11-29 16:48 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
    > On 29/11/2017 07:07, Wanpeng Li wrote:
    >> From: Wanpeng Li <wanpeng.li@hotmail.com>
    >>
    >> MSR_IA32_DEBUGCTLMSR is zeroed on VMEXIT, so it is saved/restored
    >> each time during world switch. Jim from Google pointed out that
    >> when running schbench in L2, vmx_vcpu_run will occupy 4% cpu time,
    >> and the 25% of vmx_vcpu_run cpu time is occupied by get_debugctlmsr().
    >> This patch caches the host IA32_DEBUGCTL MSR and saves/restores
    >> the host IA32_DEBUGCTL msr when guest/host switches to avoid to
    >> save/restore each time during world switch.
    >>
    >> Suggested-by: Jim Mattson <jmattson@google.com>
    >> Cc: Jim Mattson <jmattson@google.com>
    >> Cc: Paolo Bonzini <pbonzini@redhat.com>
    >> Cc: Radim Krčmář <rkrcmar@redhat.com>
    >> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
    >
    > The update_debugctlmsr should stay in vmx_vcpu_run so that tracing
    > features work correctly. However, the get_debugctlmsr indeed can be

    The tracing can't run except vCPU is schedule out, so why
    update_debugctlmsr should stay in vmx_vcpu_run?

    Regards,
    Wanpeng Li

    > moved to vmx_vcpu_load.
    >
    >> ---
    >> arch/x86/include/asm/kvm_host.h | 1 +
    >> arch/x86/kvm/vmx.c | 11 +++++------
    >> 2 files changed, 6 insertions(+), 6 deletions(-)
    >>
    >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    >> index 63d34bc..c904250 100644
    >> --- a/arch/x86/include/asm/kvm_host.h
    >> +++ b/arch/x86/include/asm/kvm_host.h
    >> @@ -619,6 +619,7 @@ struct kvm_vcpu_arch {
    >> unsigned long dr7;
    >> unsigned long eff_db[KVM_NR_DB_REGS];
    >> unsigned long guest_debug_dr7;
    >> + unsigned long debugctlmsr;
    >
    > Please rename to host_debugctlmsr and place it in struct vcpu_vmx.
    >
    > Thanks,
    >
    > Paolo
    >
    >> u64 msr_platform_info;
    >> u64 msr_misc_features_enables;
    >>
    >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
    >> index 8c7e816..b167bba 100644
    >> --- a/arch/x86/kvm/vmx.c
    >> +++ b/arch/x86/kvm/vmx.c
    >> @@ -2326,6 +2326,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
    >>
    >> vmx_vcpu_pi_load(vcpu, cpu);
    >> vmx->host_pkru = read_pkru();
    >> + vcpu->arch.debugctlmsr = get_debugctlmsr();
    >> }
    >>
    >> static void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
    >> @@ -2347,6 +2348,9 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
    >> vmx_vcpu_pi_put(vcpu);
    >>
    >> __vmx_load_host_state(to_vmx(vcpu));
    >> + /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
    >> + if (vcpu->arch.debugctlmsr)
    >> + update_debugctlmsr(vcpu->arch.debugctlmsr);
    >> }
    >>
    >> static bool emulation_required(struct kvm_vcpu *vcpu)
    >> @@ -9346,7 +9350,7 @@ static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
    >> static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
    >> {
    >> struct vcpu_vmx *vmx = to_vmx(vcpu);
    >> - unsigned long debugctlmsr, cr3, cr4;
    >> + unsigned long cr3, cr4;
    >>
    >> /* Record the guest's net vcpu time for enforced NMI injections. */
    >> if (unlikely(!enable_vnmi &&
    >> @@ -9399,7 +9403,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
    >> __write_pkru(vcpu->arch.pkru);
    >>
    >> atomic_switch_perf_msrs(vmx);
    >> - debugctlmsr = get_debugctlmsr();
    >>
    >> vmx_arm_hv_timer(vcpu);
    >>
    >> @@ -9509,10 +9512,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
    >> #endif
    >> );
    >>
    >> - /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
    >> - if (debugctlmsr)
    >> - update_debugctlmsr(debugctlmsr);
    >> -
    >> #ifndef CONFIG_X86_64
    >> /*
    >> * The sysexit path does not restore ds/es, so we must set them to
    >>
    >

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