Re: [RFC PATCH v5 076/104] KVM: x86: Add option to force LAPIC expiration wait

[Sean Christopherson]

On Fri, Mar 04, 2022, isaku.yamahata@xxxxxxxxx wrote:
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> 
> Add an option to skip the IRR check-in kvm_wait_lapic_expire().  This
> will be used by TDX to wait if there is an outstanding notification for
> a TD, i.e. a virtual interrupt is being triggered via posted interrupt
> processing.  KVM TDX doesn't emulate PI processing, i.e. there will
> never be a bit set in IRR/ISR, so the default behavior for APICv of
> querying the IRR doesn't work as intended.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  arch/x86/kvm/lapic.c   | 4 ++--
>  arch/x86/kvm/lapic.h   | 2 +-
>  arch/x86/kvm/svm/svm.c | 2 +-
>  arch/x86/kvm/vmx/vmx.c | 2 +-
>  4 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9322e6340a74..d49f029ef0e3 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1620,12 +1620,12 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>  		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
>  }
>  
> -void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> +void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu, bool force_wait)
>  {
>  	if (lapic_in_kernel(vcpu) &&
>  	    vcpu->arch.apic->lapic_timer.expired_tscdeadline &&
>  	    vcpu->arch.apic->lapic_timer.timer_advance_ns &&
> -	    lapic_timer_int_injected(vcpu))
> +	    (force_wait || lapic_timer_int_injected(vcpu)))
>  		__kvm_wait_lapic_expire(vcpu);

If the guest_apic_protected idea works, rather than require TDX to tell the local
APIC that it should wait, the common code can instead assume a timer IRQ is pending
if the IRR holds garbage.

Again, compile tested only...

From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Fri, 8 Apr 2022 09:24:39 -0700
Subject: [PATCH] KVM: x86: Assume timer IRQ was injected if APIC state is
 proteced

If APIC state is protected, i.e. the vCPU is a TDX guest, assume a timer
IRQ was injected when deciding whether or not to busy wait in the "timer
advanced" path.  The "real" vIRR is not readable/writable, so trying to
query for a pending timer IRQ will return garbage.

Note, TDX can scour the PIR if it wants to be more precise and skip the
"wait" call entirely.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/lapic.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 50a483abc0fe..e5555dce8db8 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1531,8 +1531,17 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
 static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
-	u32 reg = kvm_lapic_get_reg(apic, APIC_LVTT);
+	u32 reg;

+	/*
+	 * Assume a timer IRQ was "injected" if the APIC is protected.  KVM's
+	 * copy of the vIRR is bogus, it's the responsibility of the caller to
+	 * precisely check whether or not a timer IRQ is pending.
+	 */
+	if (apic->guest_apic_protected)
+		return true;
+
+	reg  = kvm_lapic_get_reg(apic, APIC_LVTT);
 	if (kvm_apic_hw_enabled(apic)) {
 		int vec = reg & APIC_VECTOR_MASK;
 		void *bitmap = apic->regs + APIC_ISR;

base-commit: 33f2439cd63c84fcbc8b4cdd4eb731e83deead90
--