KVM: x86: Move handling of is_guest_mode() into fastpath exit handlers

Let the fastpath code decide which exits can/can't be handled in the
fastpath when L2 is active, e.g. when KVM generates a VMX preemption
timer exit to forcefully regain control, there is no "work" to be done and
so such exits can be handled in the fastpath regardless of whether L1 or
L2 is active.

Moving the is_guest_mode() check into the fastpath code also makes it
easier to see that L2 isn't allowed to use the fastpath in most cases,
e.g. it's not immediately obvious why handle_fastpath_preemption_timer()
is called from the fastpath and the normal path.

Link: https://lore.kernel.org/r/20240110012705.506918-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ab8ad8d0e818070e8e886df0b7aac982b337f880..fe4ac03aaa3a81b062d59335a9e02c341f0bf567 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4089,6 +4089,9 @@ static int svm_vcpu_pre_run(struct kvm_vcpu *vcpu)
 
 static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 {
+       if (is_guest_mode(vcpu))
+               return EXIT_FASTPATH_NONE;
+
        if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
            to_svm(vcpu)->vmcb->control.exit_info_1)
                return handle_fastpath_set_msr_irqoff(vcpu);
@@ -4235,9 +4238,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
 
        svm_complete_interrupts(vcpu);
 
-       if (is_guest_mode(vcpu))
-               return EXIT_FASTPATH_NONE;
-
        return svm_exit_handlers_fastpath(vcpu);
 }
 

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 7bd360b5bfb95e414054bf22c89fc9b373d3c8a4..6f21d389bed5b1ece23c703e2a4ea54828dddbaf 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7214,6 +7214,9 @@ void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
 
 static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 {
+       if (is_guest_mode(vcpu))
+               return EXIT_FASTPATH_NONE;
+
        switch (to_vmx(vcpu)->exit_reason.basic) {
        case EXIT_REASON_MSR_WRITE:
                return handle_fastpath_set_msr_irqoff(vcpu);
@@ -7425,9 +7428,6 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
        vmx_recover_nmi_blocking(vmx);
        vmx_complete_interrupts(vmx);
 
-       if (is_guest_mode(vcpu))
-               return EXIT_FASTPATH_NONE;
-
        return vmx_exit_handlers_fastpath(vcpu);
 }