Re: [RFC PATCH v3 01/16] KVM: arm64: Initialize VCPU mdcr_el2 before loading it
From: James Morse <james.morse@arm.com>
Date: 2020-11-19 18:08:51
Also in:
kvmarm
Hi Alex, On 27/10/2020 17:26, Alexandru Elisei wrote:
When a VCPU is created, the kvm_vcpu struct is initialized to zero in kvm_vm_ioctl_create_vcpu(). On VHE systems, the first time vcpu.arch.mdcr_el2 is loaded on hardware is in vcpu_load(), before it is set to a sensible value in kvm_arm_setup_debug() later in the run loop. The result is that KVM executes for a short time with MDCR_EL2 set to zero. This is mostly harmless as we don't need to trap debug and SPE register accesses from EL1 (we're still running in the host at EL2), but we do set MDCR_EL2.HPMN to 0 which is constrained unpredictable according to ARM DDI 0487F.b, page D13-3620; the required behavior from the hardware in this case is to reserve an unkown number of registers for EL2 and EL3 exclusive use. Initialize mdcr_el2 in kvm_vcpu_vcpu_first_run_init(), so we can avoid the constrained unpredictable behavior and to ensure that the MDCR_EL2 register has the same value after each vcpu_load(), including the first time the VCPU is run.
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c index 7a7e425616b5..22ee448aee2b 100644 --- a/arch/arm64/kvm/debug.c +++ b/arch/arm64/kvm/debug.c@@ -68,6 +68,59 @@ void kvm_arm_init_debug(void)
+static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu, u32 host_mdcr)
+{
+ bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
+
+ /*
+ * This also clears MDCR_EL2_E2PB_MASK to disable guest access
+ * to the profiling buffer.
+ */
+ vcpu->arch.mdcr_el2 = host_mdcr & MDCR_EL2_HPMN_MASK;
+ vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
+ MDCR_EL2_TPMS |
+ MDCR_EL2_TPMCR |
+ MDCR_EL2_TDRA |
+ MDCR_EL2_TDOSA);+ if (vcpu->guest_debug) {
+ /* Route all software debug exceptions to EL2 */
+ vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
+ if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
+ trap_debug = true;
+ }This had me confused for a while... could you hint that this is when the guest is being 'external' debugged by the VMM? (its clear-er before this change) Thanks, James
+ /* Trap debug register access */
+ if (trap_debug)
+ vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
+
+ trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
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