It recently appeared that, whien running VHE, there is a notable difference between using CNTKCTL_EL1 and CNTHCTL_EL2, despite what the architecture documents:
- When accessed from EL2, bits [19:18] and [16:10] same bits have the same assignment as CNTHCTL_EL2 - When accessed from EL1, bits [19:18] and [16:10] are RES0
It is all OK, until you factor in NV, where the EL2 guest runs at EL1. In this configuration, CNTKCTL_EL11 doesn't trap, nor ends up in the VNCR page. This means that any write from the guest affecting CNTHCTL_EL2 using CNTKCTL_EL1 ends up losing some state. Not good.
The fix it obvious: don't use CNTKCTL_EL1 if you want to change bits that are not part of the EL1 definition of CNTKCTL_EL1, and use CNTHCTL_EL2 instead. This doesn't change anything for a bare-metal OS, and fixes it when running under NV. The NV hypervisor will itself have to work harder to merge the two accessors.
Note that there is a pending update to the architecture to address this issue by making the affected bits UNKNOWN when CNTKCTL_EL1 is user from EL2 with VHE enabled.
Fixes: c605ee245097 ("KVM: arm64: timers: Allow physical offset without CNTPOFF_EL2") Signed-off-by: Marc Zyngier maz@kernel.org Cc: stable@vger.kernel.org # v6.4 --- arch/arm64/kvm/arch_timer.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c index 0696732fa38c..6dcdae4d38cb 100644 --- a/arch/arm64/kvm/arch_timer.c +++ b/arch/arm64/kvm/arch_timer.c @@ -827,8 +827,8 @@ static void timer_set_traps(struct kvm_vcpu *vcpu, struct timer_map *map) assign_clear_set_bit(tpt, CNTHCTL_EL1PCEN << 10, set, clr); assign_clear_set_bit(tpc, CNTHCTL_EL1PCTEN << 10, set, clr);
- /* This only happens on VHE, so use the CNTKCTL_EL1 accessor */ - sysreg_clear_set(cntkctl_el1, clr, set); + /* This only happens on VHE, so use the CNTHCTL_EL2 accessor. */ + sysreg_clear_set(cnthctl_el2, clr, set); }
void kvm_timer_vcpu_load(struct kvm_vcpu *vcpu) @@ -1563,7 +1563,7 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu) void kvm_timer_init_vhe(void) { if (cpus_have_final_cap(ARM64_HAS_ECV_CNTPOFF)) - sysreg_clear_set(cntkctl_el1, 0, CNTHCTL_ECV); + sysreg_clear_set(cnthctl_el2, 0, CNTHCTL_ECV); }
int kvm_arm_timer_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)