Re: [PATCH v2 02/12] KVM: arm64: nv: Sync nested timer state with FEAT_NV2

Volodymyr Babchuk
2025-01-26T15:25:43Z

Hi Marc,

Thank you for these patches. We (myself and Dmytro Terletskyi) are
trying to use this series to launch up Xen on Amazon Graviton 4 platform.
Graviton 4 is built on Neoverse V2 cores and does **not** support
FEAT_ECV. Looks like we have found issue in this particular patch on
this particular setup.

Marc Zyngier <maz@kernel.org> writes:

> Emulating the timers with FEAT_NV2 is a bit odd, as the timers
> can be reconfigured behind our back without the hypervisor even
> noticing. In the VHE case, that's an actual regression in the
> architecture...
>
> Co-developed-by: Christoffer Dall <christoffer.dall@arm.com>
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/kvm/arch_timer.c | 44 ++++++++++++++++++++++++++++++++++++
> arch/arm64/kvm/arm.c | 3 +++
> include/kvm/arm_arch_timer.h | 1 +
> 3 files changed, 48 insertions(+)
>
> diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
> index 1215df5904185..ee5f732fbbece 100644
> --- a/arch/arm64/kvm/arch_timer.c
> +++ b/arch/arm64/kvm/arch_timer.c
> @@ -905,6 +905,50 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
> kvm_timer_blocking(vcpu);
> }
>
> +void kvm_timer_sync_nested(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * When NV2 is on, guest hypervisors have their EL1 timer register
> + * accesses redirected to the VNCR page. Any guest action taken on
> + * the timer is postponed until the next exit, leading to a very
> + * poor quality of emulation.
> + */
> + if (!is_hyp_ctxt(vcpu))
> + return;
> +
> + if (!vcpu_el2_e2h_is_set(vcpu)) {
> + /*
> + * A non-VHE guest hypervisor doesn't have any direct access
> + * to its timers: the EL2 registers trap (and the HW is
> + * fully emulated), while the EL0 registers access memory
> + * despite the access being notionally direct. Boo.
> + *
> + * We update the hardware timer registers with the
> + * latest value written by the guest to the VNCR page
> + * and let the hardware take care of the rest.
> + */
> + write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTV_CTL_EL0), SYS_CNTV_CTL);
> + write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTV_CVAL_EL0), SYS_CNTV_CVAL);
> + write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTP_CTL_EL0), SYS_CNTP_CTL);
> + write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTP_CVAL_EL0), SYS_CNTP_CVAL);

Here you are overwriting trapped/emulated state of EL2 vtimer with EL0
vtimer, which renders all writes to EL2 timer registers useless.

This is the behavior we observed:

1. Xen writes to CNTHP_CVAL_EL2, which is trapped and handled in
kvm_arm_timer_write_sysreg().

2. timer_set_cval() updates __vcpu_sys_reg(vcpu, CNTHP_CVAL_EL2)

3. timer_restore_state() updates real CNTP_CVAL_EL0 with value from
__vcpu_sys_reg(vcpu, CNTHP_CVAL_EL2)

(so far so good)

4. kvm_timer_sync_nested() is called and it updates real CNTP_CVAL_EL0
with __vcpu_sys_reg(vcpu, CNTP_CVAL_EL0), overwriting value that we got
from Xen.

The same stands for other hypervisor timer registers of course.

I am wondering, what is the correct fix for this issue?

Also, we are observing issues with timers in Dom0, which seems related
to this, but we didn't pinpoint exact problem yet.

--
WBR, Volodymyr