Subject: Re: [PATCH 07/14] KVM: Don't block+unblock when halt-polling is successful

From: Marc Zyngier <[email-masked]>
Date: Thu, 30 Sep 2021 10:36:19 +0100

On Tue, 28 Sep 2021 17:21:12 +0100,
Sean Christopherson <[email-masked]> wrote:
> 
> On Tue, Sep 28, 2021, Marc Zyngier wrote:
> > On Mon, 27 Sep 2021 18:28:14 +0100,
> > Sean Christopherson <[email-masked]> wrote:
> > > 
> > > On Sun, Sep 26, 2021, Marc Zyngier wrote:
> > > > On Sun, 26 Sep 2021 07:27:28 +0100,
> > > > Paolo Bonzini <[email-masked]> wrote:
> > > > > 
> > > > > On 25/09/21 11:50, Marc Zyngier wrote:
> > > > > >> there is no need for arm64 to put/load
> > > > > >> the vGIC as KVM hasn't relinquished control of the vCPU in any way.
> > > > > > 
> > > > > > This doesn't mean that there is no requirement for any state
> > > > > > change. The put/load on GICv4 is crucial for performance, and the VMCR
> > > > > > resync is a correctness requirement.
> > > 
> > > Ah crud, I didn't blame that code beforehand, I simply assumed
> > > kvm_arch_vcpu_blocking() was purely for the blocking/schedule()
> > > sequence.  The comment in arm64's kvm_arch_vcpu_blocking() about
> > > kvm_arch_vcpu_runnable() makes more sense now too.
> > > 
> > > > > I wouldn't even say it's crucial for performance: halt polling cannot
> > > > > work and is a waste of time without (the current implementation of)
> > > > > put/load.
> > > > 
> > > > Not quite. A non-V{LPI,SGI} could still be used as the a wake-up from
> > > > WFI (which is the only reason we end-up on this path). Only LPIs (and
> > > > SGIs on GICv4.1) can be directly injected, meaning that SPIs and PPIs
> > > > still follow the standard SW injection model.
> > > > 
> > > > However, there is still the ICH_VMCR_EL2 requirement (to get the
> > > > up-to-date priority mask and group enable bits) for SW-injected
> > > > interrupt wake-up to work correctly, and I really don't want to save
> > > > that one eagerly on each shallow exit.
> > > 
> > > IIUC, VMCR is resident in hardware while the guest is running, and
> > > KVM needs to retrieve the VMCR when processing interrupts to
> > > determine if a interrupt is above the priority threshold.  If that's
> > > the case, then IMO handling the VMCR via an arch hook is
> > > unnecessarily fragile, e.g. any generic call that leads to
> > > kvm_arch_vcpu_runnable() needs to know that arm64 lazily retrieves a
> > > guest register.
> > 
> > Not quite. We only need to retrieve the VMCR if we are in a situation
> > where we need to trigger a wake-up from WFI at the point where we have
> > not done a vcpu_put() yet. All the other cases where the interrupt is
> > injected are managed by the HW. And the only case where
> > kvm_arch_vcpu_runnable() gets called is when blocking.
> > 
> > I also don't get why a hook would be fragile, as long as it has well
> > defined semantics.
> 
> Generic KVM should not have to know that a seemingly benign arch hook,
> kvm_arch_vcpu_runnable(), cannot be safely called without first calling another
> arch hook.  E.g. I suspect there's a (benign?) race in kvm_vcpu_on_spin().  If
> the loop is delayed between checking rcuwait_active() and vcpu_dy_runnable(),
> and the target vCPU is awakened during that period, KVM can call
> kvm_arch_vcpu_runnable() while the vCPU is running.

Humph. Indeed, there is a potential gold-plated turd there.

> It's kind of a counter-example to my below suggestion as putting the vGIC would
> indeed lead to state corruption if the vCPU is running, but I would argue that
> arm64 should override kvm_arch_dy_runnable() so that its correctness is guaranteed,
> e.g. by not calling kvm_arch_vcpu_runnable() if the vCPU is already running.

I'll work something out for that case.

> > > A better approach for VMCR would be to retrieve the value from
> > > hardware on-demand, e.g. via a hook in vgic_get_vmcr(), so that it's all but
> > > impossible to have bugs where KVM is working with a stale VMCR, e.g.
> > > 
> > > diff --git a/arch/arm64/kvm/vgic/vgic-mmio.c b/arch/arm64/kvm/vgic/vgic-mmio.c
> > > index 48c6067fc5ec..0784de0c4080 100644
> > > --- a/arch/arm64/kvm/vgic/vgic-mmio.c
> > > +++ b/arch/arm64/kvm/vgic/vgic-mmio.c
> > > @@ -828,6 +828,13 @@ void vgic_set_vmcr(struct kvm_vcpu *vcpu, struct vgic_vmcr *vmcr)
> > >  
> > >  void vgic_get_vmcr(struct kvm_vcpu *vcpu, struct vgic_vmcr *vmcr)
> > >  {
> > > +       if (!vcpu->...->vmcr_available) {
> > > +               preempt_disable();
> > > +               kvm_vgic_vmcr_sync(vcpu);
> > > +               preempt_enable();
> > > +               vcpu->...->vmcr_available = true;
> > > +       }
> > > +
> > 
> > But most of the uses of vgic_get_vmcr() are in contexts where the vcpu
> > isn't running at all (such as save/restore). It really only operates
> > on the shadow state, and what you have above will only lead to state
> > corruption.
> 
> Ignoring the kvm_arch_dy_runnable() case for the moment, how would
> it lead to corruption?  The idea is that the 'vmcr_available' flag
> would be cleared when the vCPU is run, i.e. it tracks whether or not
> the shadow state may be stale.

I guess that 'vmcr_available' would have to be initialised to 'true'
at vcpu reset time so that the userspace side cannot trigger a read
from the HW.

> > >         if (kvm_vgic_global_state.type == VGIC_V2)
> > >                 vgic_v2_get_vmcr(vcpu, vmcr);
> > >         else
> > > 
> > > 
> > > Regarding vGIC v4, does KVM require it to be resident in hardware
> > > while the vCPU is loaded?
> > 
> > It is a requirement. Otherwise, we end-up with an inconsistent state
> > between the delivery of doorbells and the state of the vgic.
> 
> For my own understanding, does KVM require it to be resident in
> hardware while the vCPU is loaded but _not_ running?  What I don't
> fully understand is how KVM can safely load/put the vCPU if that
> true, i.e. wouldn't there always be a window for badness?

No, that part is fine. It is when you start running the vcpu without
the GICv4 context loaded that ugly stuff happens (get a doorbell that
tells you to schedule the currently running vcpu, for example).

> 
> > Also, reloading the GICv4 state can be pretty expensive (multiple MMIO
> > accesses), which is why we really don't want to do that on the hot path
> > (kvm_arch_vcpu_ioctl_run() *is* a hot path).
> 
> I wasn't suggesting to reload GICv4 on every entry, it would only be reloaded
> if it was put between vcpu_load() and entry to the guest.
> 
> > > If not, then we could do something like
> > > this, which would eliminate the arch hooks entirely if the VMCR is
> > > handled as above.
> 
> ...
> 
> > > @@ -813,6 +787,13 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > >                  */
> > >                 preempt_disable();
> > > 
> > > +               /*
> > > +                * Reload vGIC v4 if necessary, as it may be put on-demand so
> > > +                * that KVM can detect directly injected interrupts, e.g. when
> > > +                * determining if the vCPU is runnable due to a pending event.
> > > +                */
> > > +               vgic_v4_load(vcpu);
> > 
> > You'd need to detect that a previous put has been done.
> 
> Not that it will likely matter, but doesn't the its_vpe.resident
> check automatically handle this?

Sort of. I eventually want to get rid of this as it papers over all
sort of sins. I introduced it exactly because of the nesting that
vcpu_block triggers, but this is a bit of a layering violation between
KVM and the underlying GICv4 driver.

> 
> > But overall, it puts the complexity at the wrong place. WFI (aka
> > kvm_vcpu_block) is the place where we want to handle this synchronisation,
> > and not the run loop.
> > 
> > Instead of having a well defined interface with the blocking code
> > where we implement the required synchronisation, you spray the vgic
> > crap all over, and it becomes much harder to reason about it. Guess
> > what, I'm not keen on it.
> 
> My objection to the arch hooks is that, from generic KVM's
> perspective, the direct dependency is not on blocking, it's on
> calling kvm_arch_vcpu_runnable().  That's why I suggested handling
> this by tracking whether or not the VMCR is up-to-date/stale, as it
> allows generic KVM to safely call kvm_arch_vcpu_runnable() whenever
> the vCPU is loaded.
> 
> I don't have a strong opinion on arm64 preferring the sync to be
> specific to WFI, but if that's the case then IMO this should be
> handled fully in arm64, e.g.  a patch like so (or with a wrapper
> around the call to kvm_vcpu_block() if we want to guard against
> future calls into generic KVM)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index fe102cd2e518..312f3acd3ca3 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -367,27 +367,12 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
> 
>  void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
>  {
> -       /*
> -        * If we're about to block (most likely because we've just hit a
> -        * WFI), we need to sync back the state of the GIC CPU interface
> -        * so that we have the latest PMR and group enables. This ensures
> -        * that kvm_arch_vcpu_runnable has up-to-date data to decide
> -        * whether we have pending interrupts.
> -        *
> -        * For the same reason, we want to tell GICv4 that we need
> -        * doorbells to be signalled, should an interrupt become pending.
> -        */
> -       preempt_disable();
> -       kvm_vgic_vmcr_sync(vcpu);
> -       vgic_v4_put(vcpu, true);
> -       preempt_enable();
> +
>  }
> 
>  void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
>  {
> -       preempt_disable();
> -       vgic_v4_load(vcpu);
> -       preempt_enable();
> +
>  }
> 
>  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 275a27368a04..9870e824a27e 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -95,8 +95,28 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
>         } else {
>                 trace_kvm_wfx_arm64(*vcpu_pc(vcpu), false);
>                 vcpu->stat.wfi_exit_stat++;
> +
> +               /*
> +                * Sync back the state of the GIC CPU interface so that we have
> +                * the latest PMR and group enables. This ensures that
> +                * kvm_arch_vcpu_runnable has up-to-date data to decide whether
> +                * we have pending interrupts, e.g. when determining if the
> +                * vCPU should block.
> +                *
> +                * For the same reason, we want to tell GICv4 that we need
> +                * doorbells to be signalled, should an interrupt become pending.
> +                */
> +               preempt_disable();
> +               kvm_vgic_vmcr_sync(vcpu);
> +               vgic_v4_put(vcpu, true);
> +               preempt_enable();
> +
>                 kvm_vcpu_block(vcpu);
>                 kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> +
> +               preempt_disable();
> +               vgic_v4_load(vcpu);
> +               preempt_enable();
>         }
> 
>         kvm_incr_pc(vcpu);

I actually largely prefer this approach, which is massively more
readable than the current setup. Feel free to wrap that in your
series.

I'll also have a look at the vcpu_dy_runnable() asap.

Thanks,

	M.

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

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