Subject: [PATCH v2 89/94] KVM: arm64: nv: Synchronize PSTATE early on exit

From: Marc Zyngier <[email-masked]>
Date: Tue, 11 Feb 2020 17:49:33 +0000

The NV code relies on predicates such as is_hyp_ctxt() being
reliable. In turn, is_hyp_ctxt() relies on things like PSTATE
and the virtual HCR_EL2 being accurate.

But with ARMv8.4-NV removing trapping for a large part of the
EL2 system registers (among which HCR_EL2), we can't use such
trapping to synchronize the rest of the state.

Let's look at the following sequence for a VHE guest:

 (1) enter guest in host EL0
 (2) guest traps to guest vEL2 (no hypervisor intervention)
 (3) guest clears virtual HCR_EL2.TGE (no trap either)
 (4) host interrupt fires, exit
 (5) is_hyp_ctxt() now says "guest" (PSTATE.M==EL1 and TGE==0)

It is obvious that such behaviour would be rather unfortunate,
and lead to interesting, difficult to catch bugs specially if
preemption kicks in (yes, I wasted a whole week chasing this one).

In order to preserve the invariant that a guest entered in host
context must exit in the same context, we must make sure that
is_hyp_ctxt() works correctly. Since we can always observe the
guest value of HCR_EL2.{E2H,TGE} in the VNCR_EL2 page, we solely
need to synchronize PSTATE as early as possible.

This basically amounts to moving from_hw_pstate() as close
as possible to the guest exit point, and fixup_guest_exit()
seems as good a place as any.

Signed-off-by: Marc Zyngier <[email-masked]>
---
 arch/arm64/kvm/hyp/switch.c    | 51 ++++++++++++++++++++++++++++++++--
 arch/arm64/kvm/hyp/sysreg-sr.c | 26 ++---------------
 2 files changed, 51 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 3deea28794e1..c35e67241d8e 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -537,8 +537,45 @@ static bool __hyp_text handle_tx2_tvm(struct kvm_vcpu *vcpu)
  * the guest, false when we should restore the host state and return to the
  * main run loop.
  */
-static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
+static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code,
+					bool hyp_ctxt)
 {
+	/*
+	 * Sync pstate back as early as possible, so that is_hyp_ctxt()
+	 * reflects the exact context. It is otherwise possible to get
+	 * confused with a VHE guest and ARMv8.4-NV, such as:
+	 *
+	 * (1) enter guest in host EL0
+	 * (2) guest traps to guest vEL2 (no hypervisor intervention)
+	 * (3) guest clears virtual HCR_EL2.TGE (no trap either)
+	 * (4) host interrupt fires, exit
+	 * (5) is_hyp_ctxt() now says "guest" (pstate.M==EL1 and TGE==0)
+	 *
+	 * If host preemption occurs, vcpu_load/put() will be very confused.
+	 * This of course only matters to VHE.
+	 */
+
+	if (has_vhe()) {
+		u64 pstate = read_sysreg_el2(SYS_SPSR);
+
+		if (unlikely(hyp_ctxt)) {
+			u64 mode = pstate & PSR_MODE_MASK;
+
+			switch (mode) {
+			case PSR_MODE_EL1t:
+				mode = PSR_MODE_EL2t;
+				break;
+			case PSR_MODE_EL1h:
+				mode = PSR_MODE_EL2h;
+				break;
+			}
+
+			pstate = (pstate & ~PSR_MODE_MASK) | mode;
+		}
+
+		*vcpu_cpsr(vcpu) = pstate;
+	}
+
 	if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
 		vcpu->arch.fault.esr_el2 = read_sysreg_el2(SYS_ESR);
 
@@ -683,6 +720,7 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 	struct kvm_cpu_context *host_ctxt;
 	struct kvm_cpu_context *guest_ctxt;
 	u64 exit_code;
+	bool hyp_ctxt;
 
 	host_ctxt = vcpu->arch.host_cpu_context;
 	host_ctxt->__hyp_running_vcpu = vcpu;
@@ -709,12 +747,19 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 
 	__set_guest_arch_workaround_state(vcpu);
 
+	/*
+	 * Being in HYP context or not is an invariant here. If we enter in
+	 * a given context, we exit in the same context. We can thus only
+	 * sample the context once.
+	 */
+	hyp_ctxt = is_hyp_ctxt(vcpu);
+
 	do {
 		/* Jump in the fire! */
 		exit_code = __guest_enter(vcpu, host_ctxt);
 
 		/* And we're baaack! */
-	} while (fixup_guest_exit(vcpu, &exit_code));
+	} while (fixup_guest_exit(vcpu, &exit_code, hyp_ctxt));
 
 	__set_host_arch_workaround_state(vcpu);
 
@@ -788,7 +833,7 @@ int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
 		exit_code = __guest_enter(vcpu, host_ctxt);
 
 		/* And we're baaack! */
-	} while (fixup_guest_exit(vcpu, &exit_code));
+	} while (fixup_guest_exit(vcpu, &exit_code, false));
 
 	__set_host_arch_workaround_state(vcpu);
 
diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
index eaf78e9b3238..53ece83bf062 100644
--- a/arch/arm64/kvm/hyp/sysreg-sr.c
+++ b/arch/arm64/kvm/hyp/sysreg-sr.c
@@ -109,32 +109,12 @@ static void __hyp_text __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
 		__sysreg_save_vel1_state(ctxt);
 }
 
-static u64 __hyp_text from_hw_pstate(const struct kvm_cpu_context *ctxt)
-{
-	u64 reg = read_sysreg_el2(SYS_SPSR);
-
-	if (__is_hyp_ctxt(ctxt)) {
-		u64 mode = reg & (PSR_MODE_MASK | PSR_MODE32_BIT);
-
-		switch (mode) {
-		case PSR_MODE_EL1t:
-			mode = PSR_MODE_EL2t;
-			break;
-		case PSR_MODE_EL1h:
-			mode = PSR_MODE_EL2h;
-			break;
-		}
-
-		return (reg & ~(PSR_MODE_MASK | PSR_MODE32_BIT)) | mode;
-	}
-
-	return reg;
-}
-
 static void __hyp_text __sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
 {
+	/* On VHE, PSTATE is saved in fixup_guest_exit() */
+	if (!has_vhe())
+		ctxt->regs.pstate 	= read_sysreg_el2(SYS_SPSR);
 	ctxt->regs.pc			= read_sysreg_el2(SYS_ELR);
-	ctxt->regs.pstate		= from_hw_pstate(ctxt);
 
 	if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN))
 		ctxt_sys_reg(ctxt, DISR_EL1) = read_sysreg_s(SYS_VDISR_EL2);
-- 
2.20.1

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