KVM: arm64: nv: Exclude guest's TWED configuration when TWE isn't set

oupton · Marc Zyngier · commit 05d9f3408334 · 2025-09-19T14:01:35.000+01:00
Ignore the guest hypervisor's configured TWE delay if it hasn't actually
requested WFE traps. Otherwise, OR'ing these fields into the effective
HCR when the guest sets TWE is safe as KVM doesn't use FEAT_TWED and
leaves the fields initialized to 0.

Signed-off-by: Oliver Upton &lt;oliver.upton@linux.dev&gt;
Signed-off-by: Marc Zyngier &lt;maz@kernel.org&gt;
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -95,6 +95,13 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)
 			/* Force NV2 in case the guest is forgetful... */
 			guest_hcr |= HCR_NV2;
 		}
+
+		/*
+		 * Exclude the guest's TWED configuration if it hasn't set TWE
+		 * to avoid potentially delaying traps for the host.
+		 */
+		if (!(guest_hcr & HCR_TWE))
+			guest_hcr &= ~(HCR_EL2_TWEDEn | HCR_EL2_TWEDEL);
 	}
 
 	BUG_ON(host_data_test_flag(VCPU_IN_HYP_CONTEXT) &&

Original file line number	Diff line number	Diff line change
`@@ -95,6 +95,13 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)`
`95`	`95`	`/* Force NV2 in case the guest is forgetful... */`
`96`	`96`	`guest_hcr \|= HCR_NV2;`
`97`	`97`	`}`
	`98`	`+`
	`99`	`+ /*`
	`100`	`+ * Exclude the guest's TWED configuration if it hasn't set TWE`
	`101`	`+ * to avoid potentially delaying traps for the host.`
	`102`	`+ */`
	`103`	`+ if (!(guest_hcr & HCR_TWE))`
	`104`	`+ guest_hcr &= ~(HCR_EL2_TWEDEn \| HCR_EL2_TWEDEL);`
`98`	`105`	`}`
`99`	`106`
`100`	`107`	`BUG_ON(host_data_test_flag(VCPU_IN_HYP_CONTEXT) &&`