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

Marc Zyngier
2025-01-28T12:17:11Z

On Tue, 28 Jan 2025 11:29:18 +0000,
Volodymyr Babchuk <Volodymyr_Babchuk@epam.com> wrote:
>
>
> Hi Marc,
>
>
> Marc Zyngier <maz@kernel.org> writes:
>
> > + Wei-Lin Chang, who spotted something similar 3 weeks ago, that I
> > didn't manage to investigate in time.
> >
> > On Sun, 26 Jan 2025 15:25:39 +0000,
> > Volodymyr Babchuk <Volodymyr_Babchuk@epam.com> wrote:
> >>
> >>
> >> 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.
> >
> > Thanks for the great debug above, much appreciated.
> >
> > As Wei-Lin pointed out in their email[1], there is a copious amount of
> > nonsense here. This is due to leftovers from the mix of NV+NV2 that
> > KVM was initially trying to handle before switching to NV2 only.
> >
> > The whole VHE vs nVHE makes no sense at all, and both should have the
> > same behaviour. The only difference is around what gets trapped, and
> > what doesn't.
> >
> > Finally, this crap is masking a subtle bug in timer_emulate(), where
> > we return too early on updating the IRQ state, hence failing to
> > publish the interrupt state.
> >
> > Could you please give the hack below a go with your setup and report
> > whether it solves this particular issue?
>
> Thanks! This is exactly what we needed. Your suggested changes fixed
> both issues: in Xen and in Dom0.

Great, thanks for letting me know.

I'll shortly post the fixes on the list, and would appreciate it if
you could reply with a Tested-by: tag.

Thanks again,

M.

--
Without deviation from the norm, progress is not possible.