linux-kernel - Re: [patch V4 part 2 13/18] x86/kvm: Move context tracking where it belongs

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Date:   Wed, 6 May 2020 09:42:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 2 13/18] x86/kvm: Move context tracking where it
 belongs

On 05/05/20 15:41, Thomas Gleixner wrote:
> Context tracking for KVM happens way too early in the vcpu_run()
> code. Anything after guest_enter_irqoff() and before guest_exit_irqoff()
> cannot use RCU and should also be not instrumented.
> 
> The current way of doing this covers way too much code. Move it closer to
> the actual vmenter/exit code.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/svm/svm.c |   16 ++++++++++++++++
>  arch/x86/kvm/vmx/vmx.c |   10 ++++++++++
>  arch/x86/kvm/x86.c     |    2 --
>  3 files changed, 26 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3330,6 +3330,14 @@ static void svm_vcpu_run(struct kvm_vcpu
>  	 */
>  	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
>  
> +	/*
> +	 * Tell context tracking that this CPU is about to enter guest
> +	 * mode. This has to be after x86_spec_ctrl_set_guest() because
> +	 * that can take locks (lockdep needs RCU) and calls into world and
> +	 * some more.
> +	 */
> +	guest_enter_irqoff();
> +
>  	__svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>  
>  #ifdef CONFIG_X86_64
> @@ -3340,6 +3348,14 @@ static void svm_vcpu_run(struct kvm_vcpu
>  	loadsegment(gs, svm->host.gs);
>  #endif
>  #endif
> +	/*
> +	 * Tell context tracking that this CPU is back.
> +	 *
> +	 * This needs to be done before the below as native_read_msr()
> +	 * contains a tracepoint and x86_spec_ctrl_restore_host() calls
> +	 * into world and some more.
> +	 */
> +	guest_exit_irqoff();
>  
>  	/*
>  	 * We do not use IBRS in the kernel. If this vCPU has used the
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6603,6 +6603,11 @@ static void vmx_vcpu_run(struct kvm_vcpu
>  	 */
>  	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
>  
> +	/*
> +	 * Tell context tracking that this CPU is about to enter guest mode.
> +	 */
> +	guest_enter_irqoff();
> +
>  	/* L1D Flush includes CPU buffer clear to mitigate MDS */
>  	if (static_branch_unlikely(&vmx_l1d_should_flush))
>  		vmx_l1d_flush(vcpu);
> @@ -6618,6 +6623,11 @@ static void vmx_vcpu_run(struct kvm_vcpu
>  	vcpu->arch.cr2 = read_cr2();
>  
>  	/*
> +	 * Tell context tracking that this CPU is back.
> +	 */
> +	guest_exit_irqoff();
> +
> +	/*
>  	 * We do not use IBRS in the kernel. If this vCPU has used the
>  	 * SPEC_CTRL MSR it may have left it on; save the value and
>  	 * turn it off. This is much more efficient than blindly adding
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8404,7 +8404,6 @@ static int vcpu_enter_guest(struct kvm_v
>  	}
>  
>  	trace_kvm_entry(vcpu->vcpu_id);
> -	guest_enter_irqoff();
>  
>  	fpregs_assert_state_consistent();
>  	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> @@ -8467,7 +8466,6 @@ static int vcpu_enter_guest(struct kvm_v
>  	local_irq_disable();
>  	kvm_after_interrupt(vcpu);
>  
> -	guest_exit_irqoff();
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
>  		if (delta != S64_MIN) {
> 

Acked-by: Paolo Bonzini <pbonzini@...hat.com>