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

Volodymyr Babchuk
2025-01-28T11:29:22Z

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.

[...]

--
WBR, Volodymyr