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Subject[PATCH RFC] KVM: x86: emulate wait-for-SIPI and SIPI-VMExit
Date
From: Yadong Qi <yadong.qi@intel.com>

Background: We have a lightweight HV, it needs INIT-VMExit and
SIPI-VMExit to wake-up APs for guests since it do not monitoring
the Local APIC. But currently virtual wait-for-SIPI(WFS) state
is not supported in KVM, so when running on top of KVM, the L1
HV cannot receive the INIT-VMExit and SIPI-VMExit which cause
the L2 guest cannot wake up the APs.

This patch is incomplete, it emulated wait-for-SIPI state by halt
the vCPU and emulated SIPI-VMExit to L1 when trapped SIPI signal
from L2. I am posting it RFC to gauge whether or not upstream
KVM is interested in emulating wait-for-SIPI state before
investing the time to finish the full support.

According to Intel SDM Chapter 25.2 Other Causes of VM Exits,
SIPIs cause VM exits when a logical processor is in
wait-for-SIPI state.

In this patch:
1. introduce SIPI exit reason,
2. introduce wait-for-SIPI state for nVMX,
3. advertise wait-for-SIPI support to guest.

When L1 hypervisor is not monitoring Local APIC, L0 need to emulate
INIT-VMExit and SIPI-VMExit to L1 to emulate INIT-SIPI-SIPI for
L2. L2 LAPIC write would be traped by L0 Hypervisor(KVM), L0 should
emulate the INIT/SIPI vmexit to L1 hypervisor to set proper state
for L2's vcpu state.

Handle procdure:
Source vCPU:
L2 write LAPIC.ICR(INIT).
L0 trap LAPIC.ICR write(INIT): inject a latched INIT event to target
vCPU.
Target vCPU:
L0 emulate an INIT VMExit to L1 if is guest mode.
L1 set guest VMCS, guest_activity_state=WAIT_SIPI, vmresume.
L0 halt vCPU if (vmcs12.guest_activity_state == WAIT_SIPI).

Source vCPU:
L2 write LAPIC.ICR(SIPI).
L0 trap LAPIC.ICR write(INIT): inject a latched SIPI event to traget
vCPU.
Target vCPU:
L0 emulate an SIPI VMExit to L1 if (vmcs12.guest_activity_state ==
WAIT_SIPI).
L1 set CS:IP, guest_activity_state=ACTIVE, vmresume
L0 resume to L2
L2 start-up

Signed-off-by: Yadong Qi <yadong.qi@intel.com>
---
arch/x86/include/asm/vmx.h | 1 +
arch/x86/include/uapi/asm/vmx.h | 2 ++
arch/x86/kvm/lapic.c | 5 +++--
arch/x86/kvm/vmx/nested.c | 25 +++++++++++++++++++++----
4 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
index cd7de4b401fe..bff06dc64c52 100644
--- a/arch/x86/include/asm/vmx.h
+++ b/arch/x86/include/asm/vmx.h
@@ -113,6 +113,7 @@
#define VMX_MISC_PREEMPTION_TIMER_RATE_MASK 0x0000001f
#define VMX_MISC_SAVE_EFER_LMA 0x00000020
#define VMX_MISC_ACTIVITY_HLT 0x00000040
+#define VMX_MISC_ACTIVITY_WAIT_SIPI 0x00000100
#define VMX_MISC_ZERO_LEN_INS 0x40000000
#define VMX_MISC_MSR_LIST_MULTIPLIER 512

diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
index b8ff9e8ac0d5..ada955c5ebb6 100644
--- a/arch/x86/include/uapi/asm/vmx.h
+++ b/arch/x86/include/uapi/asm/vmx.h
@@ -32,6 +32,7 @@
#define EXIT_REASON_EXTERNAL_INTERRUPT 1
#define EXIT_REASON_TRIPLE_FAULT 2
#define EXIT_REASON_INIT_SIGNAL 3
+#define EXIT_REASON_SIPI_SIGNAL 4

#define EXIT_REASON_INTERRUPT_WINDOW 7
#define EXIT_REASON_NMI_WINDOW 8
@@ -94,6 +95,7 @@
{ EXIT_REASON_EXTERNAL_INTERRUPT, "EXTERNAL_INTERRUPT" }, \
{ EXIT_REASON_TRIPLE_FAULT, "TRIPLE_FAULT" }, \
{ EXIT_REASON_INIT_SIGNAL, "INIT_SIGNAL" }, \
+ { EXIT_REASON_SIPI_SIGNAL, "SIPI_SIGNAL" }, \
{ EXIT_REASON_INTERRUPT_WINDOW, "INTERRUPT_WINDOW" }, \
{ EXIT_REASON_NMI_WINDOW, "NMI_WINDOW" }, \
{ EXIT_REASON_TASK_SWITCH, "TASK_SWITCH" }, \
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 5ccbee7165a2..d04ac7dc6adf 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2839,7 +2839,7 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)

/*
* INITs are latched while CPU is in specific states
- * (SMM, VMX non-root mode, SVM with GIF=0).
+ * (SMM, SVM with GIF=0).
* Because a CPU cannot be in these states immediately
* after it has processed an INIT signal (and thus in
* KVM_MP_STATE_INIT_RECEIVED state), just eat SIPIs
@@ -2847,7 +2847,8 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
*/
if (kvm_vcpu_latch_init(vcpu)) {
WARN_ON_ONCE(vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED);
- if (test_bit(KVM_APIC_SIPI, &apic->pending_events))
+ if (test_bit(KVM_APIC_SIPI, &apic->pending_events) &&
+ !is_guest_mode(vcpu))
clear_bit(KVM_APIC_SIPI, &apic->pending_events);
return;
}
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 1bb6b31eb646..399933b8ac3a 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2946,7 +2946,8 @@ static int nested_vmx_check_vmcs_link_ptr(struct kvm_vcpu *vcpu,
static int nested_check_guest_non_reg_state(struct vmcs12 *vmcs12)
{
if (CC(vmcs12->guest_activity_state != GUEST_ACTIVITY_ACTIVE &&
- vmcs12->guest_activity_state != GUEST_ACTIVITY_HLT))
+ vmcs12->guest_activity_state != GUEST_ACTIVITY_HLT &&
+ vmcs12->guest_activity_state != GUEST_ACTIVITY_WAIT_SIPI))
return -EINVAL;

return 0;
@@ -3548,7 +3549,8 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
* awakened by event injection or by an NMI-window VM-exit or
* by an interrupt-window VM-exit, halt the vcpu.
*/
- if ((vmcs12->guest_activity_state == GUEST_ACTIVITY_HLT) &&
+ if (((vmcs12->guest_activity_state == GUEST_ACTIVITY_HLT) ||
+ (vmcs12->guest_activity_state == GUEST_ACTIVITY_WAIT_SIPI)) &&
!(vmcs12->vm_entry_intr_info_field & INTR_INFO_VALID_MASK) &&
!(vmcs12->cpu_based_vm_exec_control & CPU_BASED_NMI_WINDOW_EXITING) &&
!((vmcs12->cpu_based_vm_exec_control & CPU_BASED_INTR_WINDOW_EXITING) &&
@@ -3767,6 +3769,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
vmx->nested.nested_run_pending || kvm_event_needs_reinjection(vcpu);
bool mtf_pending = vmx->nested.mtf_pending;
struct kvm_lapic *apic = vcpu->arch.apic;
+ struct vmcs12 *vmcs12 = get_vmcs12(vcpu);

/*
* Clear the MTF state. If a higher priority VM-exit is delivered first,
@@ -3781,7 +3784,20 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
return -EBUSY;
nested_vmx_update_pending_dbg(vcpu);
clear_bit(KVM_APIC_INIT, &apic->pending_events);
- nested_vmx_vmexit(vcpu, EXIT_REASON_INIT_SIGNAL, 0, 0);
+ if (vmcs12->guest_activity_state != GUEST_ACTIVITY_WAIT_SIPI)
+ nested_vmx_vmexit(vcpu, EXIT_REASON_INIT_SIGNAL, 0, 0);
+ return 0;
+ }
+
+ if (lapic_in_kernel(vcpu) &&
+ test_bit(KVM_APIC_SIPI, &apic->pending_events)) {
+ if (block_nested_events)
+ return -EBUSY;
+
+ clear_bit(KVM_APIC_SIPI, &apic->pending_events);
+ if (vmcs12->guest_activity_state == GUEST_ACTIVITY_WAIT_SIPI)
+ nested_vmx_vmexit(vcpu, EXIT_REASON_SIPI_SIGNAL, 0,
+ apic->sipi_vector & 0xFFUL);
return 0;
}

@@ -6471,7 +6487,8 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
msrs->misc_low |=
MSR_IA32_VMX_MISC_VMWRITE_SHADOW_RO_FIELDS |
VMX_MISC_EMULATED_PREEMPTION_TIMER_RATE |
- VMX_MISC_ACTIVITY_HLT;
+ VMX_MISC_ACTIVITY_HLT |
+ VMX_MISC_ACTIVITY_WAIT_SIPI;
msrs->misc_high = 0;

/*
--
2.25.1
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