linux-kernel - Re: [PATCH] KVM: X86: set EXITING_GUEST

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Date:   Wed, 30 Nov 2022 14:29:37 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Jon Kohler <jon@...anix.com>
CC:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: X86: set EXITING_GUEST_MODE as soon as vCPU exits

On Tue, Nov 29, 2022 at 01:22:25PM -0500, Jon Kohler wrote:
>@@ -7031,6 +7042,18 @@ void noinstr vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
> void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
> 					unsigned int flags)
> {
>+	struct kvm_vcpu *vcpu = &vmx->vcpu;
>+
>+	/* Optimize IPI reduction by setting mode immediately after vmexit
>+	 * without a memmory barrier as this as not paired anywhere. vcpu->mode
>+	 * is will be set to OUTSIDE_GUEST_MODE in x86 common code with a memory
>+	 * barrier, after the host is done fully restoring various host states.
>+	 * Since the rdmsr and wrmsr below are expensive, this must be done
>+	 * first, so that the IPI suppression window covers the time dealing
>+	 * with fixing up SPEC_CTRL.
>+	 */
>+	vcpu->mode = EXITING_GUEST_MODE;

Does this break kvm_vcpu_kick()? IIUC, kvm_vcpu_kick() does nothing if
vcpu->mode is already EXITING_GUEST_MODE, expecting the vCPU will exit
guest mode. But ...

>+
> 	u64 hostval = this_cpu_read(x86_spec_ctrl_current);
> 
> 	if (!cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL))
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 2835bd796639..0e0d228f3fa5 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -2160,6 +2160,14 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
> 		data = kvm_read_edx_eax(vcpu);
> 		if (!handle_fastpath_set_tscdeadline(vcpu, data)) {
> 			kvm_skip_emulated_instruction(vcpu);
>+			/* Reset IN_GUEST_MODE since we're going to reenter
>+			 * guest as part of this fast path. This is done as
>+			 * an optimization without a memory barrier since
>+			 * EXITING_GUEST_MODE is also set without a memory
>+			 * barrier.
>+			 */
>+			if (vcpu->mode == EXITING_GUEST_MODE)
>+				vcpu->mode = IN_GUEST_MODE;

... the vCPU enters guest mode again.

I believe mode transition from EXITING_GUEST_MODE to IN_GUEST_MODE
directly isn't valid for current KVM.