Re: [PATCH 02/11] KVM: arm64: nv: Sync nested timer state with FEAT_NV2

Oliver Upton
2024-12-05T00:26:24Z

On Mon, Dec 02, 2024 at 05:21:25PM +0000, Marc Zyngier wrote:
> 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..81afafd62059f 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 EL0 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);
> + } else {
> + /*
> + * For a VHE guest hypervisor, the EL2 state is directly
> + * stored in the host EL0 timers, while the emulated EL0
> + * state is stored in the VNCR page. The latter could have
> + * been updated behind our back, and we must reset the
> + * emulation of the timers.
> + */

nitpick: s/host EL0/EL1/

At least in the way the architecture terms it there's no such thing as
an EL0 timer, and "host EL0" might lead one to think of ELIsInHost(EL0)

--
Thanks,
Oliver