Subject: [PATCH v3 65/66] KVM: arm64: nv: Fast-track 'InHost' exception returns

From: Marc Zyngier <[email-masked]>
Date: Thu, 10 Dec 2020 16:00:01 +0000

A significant part of the ARMv8.3-NV extension is to trap ERET
instructions so that the hypervisor gets a chance to switch
from a vEL2 L1 guest to an EL1 L2 guest.

But this also has the unfortunate consequence of trapping ERET
in unsuspecting circumstances, such as staying at vEL2 (interrupt
handling while being in the guest hypervisor), or returning to host
userspace in the case of a VHE guest.

Although we already make some effort to handle these ERET quicker
by not doing the put/load dance, it is still way too far down the
line for it to be efficient enough.

For these cases, it would ideal to ERET directly, no question asked.
Of course, we can't do that. But the next best thing is to do it as
early as possible, in fixup_guest_exit(), much as we would handle
FPSIMD exceptions.

Signed-off-by: Marc Zyngier <[email-masked]>
---
 arch/arm64/kvm/emulate-nested.c | 26 ++--------------
 arch/arm64/kvm/hyp/vhe/switch.c | 53 ++++++++++++++++++++++++++++++++-
 2 files changed, 54 insertions(+), 25 deletions(-)

diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index df4661515183..43197ff4a806 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -52,8 +52,7 @@ bool forward_nv_traps(struct kvm_vcpu *vcpu)
 
 void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
 {
-	u64 spsr, elr, mode;
-	bool direct_eret;
+	u64 spsr, elr;
 
 	/*
 	 * Forward this trap to the virtual EL2 if the virtual
@@ -62,31 +61,10 @@ void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
 	if (forward_nv_traps(vcpu))
 		return;
 
-	/*
-	 * Going through the whole put/load motions is a waste of time
-	 * if this is a VHE guest hypervisor returning to its own
-	 * userspace, or the hypervisor performing a local exception
-	 * return. No need to save/restore registers, no need to
-	 * switch S2 MMU. Just do the canonical ERET.
-	 */
-	spsr = vcpu_read_sys_reg(vcpu, SPSR_EL2);
-	mode = spsr & (PSR_MODE_MASK | PSR_MODE32_BIT);
-
-	direct_eret  = (mode == PSR_MODE_EL0t &&
-			vcpu_el2_e2h_is_set(vcpu) &&
-			vcpu_el2_tge_is_set(vcpu));
-	direct_eret |= (mode == PSR_MODE_EL2h || mode == PSR_MODE_EL2t);
-
-	if (direct_eret) {
-		*vcpu_pc(vcpu) = vcpu_read_sys_reg(vcpu, ELR_EL2);
-		*vcpu_cpsr(vcpu) = spsr;
-		trace_kvm_nested_eret(vcpu, *vcpu_pc(vcpu), spsr);
-		return;
-	}
-
 	preempt_disable();
 	kvm_arch_vcpu_put(vcpu);
 
+	spsr = __vcpu_sys_reg(vcpu, SPSR_EL2);
 	elr = __vcpu_sys_reg(vcpu, ELR_EL2);
 
 	trace_kvm_nested_eret(vcpu, elr, spsr);
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 6d57d09b8503..c90aed418f73 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -168,6 +168,56 @@ void deactivate_traps_vhe_put(void)
 	__deactivate_traps_common();
 }
 
+static bool __hyp_handle_eret(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
+	u64 spsr, mode;
+
+	/*
+	 * Going through the whole put/load motions is a waste of time
+	 * if this is a VHE guest hypervisor returning to its own
+	 * userspace, or the hypervisor performing a local exception
+	 * return. No need to save/restore registers, no need to
+	 * switch S2 MMU. Just do the canonical ERET. Unless the trap
+	 * has to be forwarded further down the line, of course...
+	 */
+	if (kvm_vcpu_trap_get_class(vcpu) != ESR_ELx_EC_ERET)
+		return false;
+
+	/*
+	 * Let the trap forwarding be handled by the normal exception
+	 * handling code.
+	 */
+	if (__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_NV)
+		return false;
+
+	spsr = read_sysreg_el1(SYS_SPSR);
+	spsr = __fixup_spsr_el2_read(ctxt, spsr);
+	mode = spsr & (PSR_MODE_MASK | PSR_MODE32_BIT);
+
+	switch (mode) {
+	case PSR_MODE_EL0t:
+		if (!(vcpu_el2_e2h_is_set(vcpu) && vcpu_el2_tge_is_set(vcpu)))
+			return false;
+		break;
+	case PSR_MODE_EL2t:
+		mode = PSR_MODE_EL1t;
+		break;
+	case PSR_MODE_EL2h:
+		mode = PSR_MODE_EL1h;
+		break;
+	default:
+		return false;
+	}
+
+	spsr = (spsr & ~(PSR_MODE_MASK | PSR_MODE32_BIT)) | mode;
+
+	write_sysreg_el2(spsr, SYS_SPSR);
+	write_sysreg_el2(read_sysreg_el1(SYS_ELR), SYS_ELR);
+
+	return true;
+}
+
 static bool fixup_guest_exit_vhe(struct kvm_vcpu *vcpu, u64 *exit_code,
 				 bool hyp_ctxt)
 {
@@ -209,7 +259,8 @@ static bool fixup_guest_exit_vhe(struct kvm_vcpu *vcpu, u64 *exit_code,
 	fixup_guest_exit_prologue(vcpu, exit_code);
 
 	if (*exit_code == ARM_EXCEPTION_TRAP) {
-		/* more to come here */
+		if (__hyp_handle_eret(vcpu))
+			return true;
 	}
 
 	return fixup_guest_exit(vcpu, exit_code);
-- 
2.29.2

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