On Wed, Feb 20, 2019 at 07:14:53PM +0000, Dave Martin wrote:
> On Fri, Jan 25, 2019 at 10:46:52AM +0100, Christoffer Dall wrote:
> > Resetting the VCPU state modifies the system register state in memory,
> > but this may interact with vcpu_load/vcpu_put if running with preemption
> > disabled, which in turn may lead to corrupted system register state.
>
> Should this be "enabled"?
>
> Too late now, but I want to make sure I understand this right for
> patches that will go on top.
>
> > Address this by disabling preemption and doing put/load if required
> > around the reset logic.
> >
> > Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxx>
> > Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> > ---
> > arch/arm64/kvm/reset.c | 26 ++++++++++++++++++++++++--
> > 1 file changed, 24 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index b72a3dd56204..f21a2a575939 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -105,16 +105,33 @@ int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> > * This function finds the right table above and sets the registers on
> > * the virtual CPU struct to their architecturally defined reset
> > * values.
> > + *
> > + * Note: This function can be called from two paths: The KVM_ARM_VCPU_INIT
> > + * ioctl or as part of handling a request issued by another VCPU in the PSCI
> > + * handling code. In the first case, the VCPU will not be loaded, and in the
> > + * second case the VCPU will be loaded. Because this function operates purely
> > + * on the memory-backed valus of system registers, we want to do a full put if
> > + * we were loaded (handling a request) and load the values back at the end of
> > + * the function. Otherwise we leave the state alone. In both cases, we
> > + * disable preemption around the vcpu reset as we would otherwise race with
> > + * preempt notifiers which also call put/load.
> > */
> > int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> > {
> > const struct kvm_regs *cpu_reset;
> > + int ret = -EINVAL;
> > + bool loaded;
> > +
> > + preempt_disable();
> > + loaded = (vcpu->cpu != -1);
> > + if (loaded)
> > + kvm_arch_vcpu_put(vcpu);
> >
> > switch (vcpu->arch.target) {
> > default:
> > if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
> > if (!cpu_has_32bit_el1())
> > - return -EINVAL;
> > + goto out;
> > cpu_reset = &default_regs_reset32;
> > } else {
> > cpu_reset = &default_regs_reset;
> > @@ -137,7 +154,12 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> > vcpu->arch.workaround_flags |= VCPU_WORKAROUND_2_FLAG;
> >
> > /* Reset timer */
> > - return kvm_timer_vcpu_reset(vcpu);
> > + ret = kvm_timer_vcpu_reset(vcpu);
> > +out:
> > + if (loaded)
> > + kvm_arch_vcpu_load(vcpu, smp_processor_id());
> > + preempt_enable();
> > + return ret;
> > }
> >
> > void kvm_set_ipa_limit(void)
>
> I was really confused by this: as far as I can see, we don't really need
> to disable preemption here once kvm_arch_vcpu_put() is complete -- at
> least not for the purpose of avoiding corruption of the reg state. But
> we _do_ need to disable the preempt notifier so that it doesn't fire
> before we are ready.
>
> It actually seems a bit surprising for a powered-off CPU to sit with the
> VM regs live and preempt notifier armed, when the vcpu thread is
> heading to interruptible sleep anyway until someone turns it on.
> Perhaps an alternative approach would be to nobble the preempt notifier
> and stick an explicit vcpu_put()...vcpu_load() around the
> swait_event_interruptible_exclusive() call in vcpu_req_sleep(). This
> is not fast path.
>
>
I think you've understood the problem correctly, and the thing here is
that we (sort-of) "abuse" disabling preemption as a way to disable
preempt notifiers, which I don't think we have. So we could add that,
and do something like:
preempt_disable();
vcpu_put(vcpu);
disable_preempt_notifiers(vcpu);
preempt_disable();
funky_stuff();
vcpu_load();
preempt_enable();
But I think that's additional complexity to get a slightly shorter
section with disabled preemption.
We could also re-architect a lot of the vcpu_load/vpcu_put functionality
more drastically, but that is difficult and requires understanding of
how the other architectures work, so at the end of the day we just use
this pattern in multiple places, which is:
preempt_disable();
vcpu_put();
modify_vcpu_state_in_memory();
vcpu_load();
preempt_enable();
Does that help?
Thanks,
Christoffer
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