[PATCH 2/8] KVM: arm64: Synchronise HCR_EL2 writes on the guest exit path

Fuad Tabba
2026-04-28T10:30:13Z

MSR HCR_EL2 is not self-synchronising. Per ARM DDI 0487 M.b K1.2.4
(p.K1-16823) and B2.6.1 (p.B2-297), a Context Synchronisation Event
is required between an HCR_EL2 write and any subsequent direct
register access at the same EL that depends on the new value being
in effect.

On the entry path, the HCR_EL2 write in __activate_traps is followed
by further EL2 sysreg work (MDCR_EL2, CPTR_EL2, VBAR_EL2, and on the
speculative-AT errata path SCTLR_EL1/TCR_EL1) before ERET into the
guest. None of those intervening accesses depend on the new HCR_EL2
value, and ERET is a CSE per ARM DDI 0487 M.b D1.4.4.1 rule RBWCFK
(p. D1-7209) conditional on SCTLR_EL2.EOS=1, which is set
unconditionally by INIT_SCTLR_EL2_MMU_ON (see the prerequisite patch
in this series). The requirement is therefore satisfied implicitly
on the activate path.

The deactivate path is different: after write_sysreg_hcr() in
__deactivate_traps() further EL2 sysreg work runs before any natural
CSE - on nVHE, __deactivate_cptr_traps and the VBAR_EL2 write; on
VHE, the timer context save which reads CNTP_CVAL_EL0 under the new
TGE/E2H, and the EL1 sysreg restore. Add an explicit isb() at each
of the two deactivate sites.

The practical impact today is bounded: HCR_EL2.E2H does not toggle
in either path, and the trap bits being changed primarily affect
EL1&0 behaviour. But the architectural rule should be honoured.
Note that write_sysreg_hcr() itself already issues isb() on the
Ampere errata path (sysreg.h), confirming the architectural
expectation; the fast path optimises that away.

The fix is at the call sites rather than inside write_sysreg_hcr()
because the macro has many users (e.g. the activate path, at.c,
hardirq.h, ptrauth alternatives) where the immediately-following
code either reaches ERET or has another CSE; making the macro emit
an unconditional ISB would impose unnecessary cost on those
well-formed users.

Fixes: 9404673293b0 ("KVM: arm64: timers: Correctly handle TGE flip with CNTPOFF_EL2")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
arch/arm64/kvm/hyp/nvhe/switch.c | 11 +++++++++++
arch/arm64/kvm/hyp/vhe/switch.c | 11 +++++++++++
2 files changed, 22 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 8d1df3d33595..9d7ead5a5503 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -105,6 +105,17 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
__deactivate_traps_common(vcpu);

write_sysreg_hcr(this_cpu_ptr(&kvm_init_params)->hcr_el2);
+ /*
+ * MSR HCR_EL2 is not self-synchronising. Per ARM ARM K1.2.4 p.K1-16823
+ * and B2.6.1 p.B2-297, a Context Synchronisation Event is required
+ * between an HCR_EL2 write and any subsequent direct register access at
+ * the same EL that depends on the new value being in effect.
+ * The activate_traps path falls through to ERET (a CSE), but the
+ * deactivate path still executes further EL2 sysreg work (CPTR/VBAR
+ * writes below) before any natural CSE, so make the synchronisation
+ * explicit.
+ */
+ isb();

__deactivate_cptr_traps(vcpu);
write_sysreg(__kvm_hyp_host_vector, vbar_el2);
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 9db3f11a4754..140d3bcb5651 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -149,6 +149,17 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
___deactivate_traps(vcpu);

write_sysreg_hcr(HCR_HOST_VHE_FLAGS);
+ /*
+ * MSR HCR_EL2 is not self-synchronising. Per ARM ARM K1.2.4 p.K1-16823
+ * and B2.6.1 p.B2-297, a Context Synchronisation Event is required
+ * between an HCR_EL2 write and any subsequent direct register access at
+ * the same EL that depends on the new value being in effect.
+ * The activate_traps path falls through to ERET (a CSE), but the
+ * deactivate path still executes further EL2 sysreg work (CPTR/VBAR
+ * writes below) before any natural CSE, so make the synchronisation
+ * explicit.
+ */
+ isb();

if (has_cntpoff()) {
struct timer_map map;
--
2.54.0.545.g6539524ca2-goog