Subject: [PATCH v2] arm/arm64: KVM: Perform local TLB invalidation when multiplexing vcpus on a single CPU

From: [email-masked] (Christoffer Dall)
Date: Tue, 1 Nov 2016 19:28:43 +0100

On Tue, Nov 01, 2016 at 04:39:29PM +0000, Marc Zyngier wrote:
> [messed up my initial reply, resending]
> 
> On Tue, Nov 01 2016 at 09:04:08 AM, Christoffer Dall
> <[email-masked]> wrote:
> > On Fri, Oct 28, 2016 at 11:27:50AM +0100, Marc Zyngier wrote:
> >> Architecturally, TLBs are private to the (physical) CPU they're
> >> associated with. But when multiple vcpus from the same VM are
> >> being multiplexed on the same CPU, the TLBs are not private
> >> to the vcpus (and are actually shared across the VMID).
> >> 
> >> Let's consider the following scenario:
> >> 
> >> - vcpu-0 maps PA to VA
> >> - vcpu-1 maps PA' to VA
> >> 
> >> If run on the same physical CPU, vcpu-1 can hit TLB entries generated
> >> by vcpu-0 accesses, and access the wrong physical page.
> >> 
> >> The solution to this is to keep a per-VM map of which vcpu ran last
> >> on each given physical CPU, and invalidate local TLBs when switching
> >> to a different vcpu from the same VM.
> >> 
> >> Reviewed-by: Mark Rutland <[email-masked]>
> >> Signed-off-by: Marc Zyngier <[email-masked]>
> >> ---
> >> Fixed comments, added Mark's RB.
> >> 
> >>  arch/arm/include/asm/kvm_host.h   | 11 ++++++++++-
> >>  arch/arm/include/asm/kvm_hyp.h    |  1 +
> >>  arch/arm/kvm/arm.c                | 35 ++++++++++++++++++++++++++++++++++-
> >>  arch/arm/kvm/hyp/switch.c         |  9 +++++++++
> >>  arch/arm64/include/asm/kvm_host.h | 11 ++++++++++-
> >>  arch/arm64/kvm/hyp/switch.c       |  8 ++++++++
> >>  6 files changed, 72 insertions(+), 3 deletions(-)
> >> 
> 
> [...]
> 
> >> @@ -310,6 +322,27 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> >>  	return 0;
> >>  }
> >>  
> >> +void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
> >> +{
> >
> > why is calling this from here sufficient?
> >
> > You only get a notification from preempt notifiers if you were preempted
> > while running (or rather while the vcpu was loaded).  I think this
> > needs
> 
> Arghh. I completely miss-read the code when writing that patch.
> 
> > to go in kvm_arch_vcpu_load, but be aware that the vcpu_load gets called
> > for other vcpu ioctls and doesn't necessarily imply that the vcpu will
> > actually run, which is also the case for the sched_in notification, btw.
> > The worst that will happen in that case is a bit of extra TLB
> > invalidation, so sticking with kvm_arch_vcpu_load is probably fine.
> 
> Indeed. I don't mind the extra invalidation, as long as it is rare
> enough. Another possibility would be to do this test on the entry path,
> once preemption is disabled.
> 
> >
> >> +	int *last_ran;
> >> +
> >> +	last_ran = per_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran, cpu);
> >> +
> >> +	/*
> >> +	 * We might get preempted before the vCPU actually runs, but
> >> +	 * this is fine. Our TLBI stays pending until we actually make
> >> +	 * it to __activate_vm, so we won't miss a TLBI. If another
> >> +	 * vCPU gets scheduled, it will see our vcpu_id in last_ran,
> >> +	 * and pend a TLBI for itself.
> >> +	 */
> >> +	if (*last_ran != vcpu->vcpu_id) {
> >> +		if (*last_ran != -1)
> >> +			vcpu->arch.tlb_vmid_stale = true;
> >> +
> >> +		*last_ran = vcpu->vcpu_id;
> >> +	}
> >> +}
> >> +
> >>  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >>  {
> >>  	vcpu->cpu = cpu;
> >> diff --git a/arch/arm/kvm/hyp/switch.c b/arch/arm/kvm/hyp/switch.c
> >> index 92678b7..a411762 100644
> >> --- a/arch/arm/kvm/hyp/switch.c
> >> +++ b/arch/arm/kvm/hyp/switch.c
> >> @@ -75,6 +75,15 @@ static void __hyp_text __activate_vm(struct kvm_vcpu *vcpu)
> >>  {
> >>  	struct kvm *kvm = kern_hyp_va(vcpu->kvm);
> >>  	write_sysreg(kvm->arch.vttbr, VTTBR);
> >> +	if (vcpu->arch.tlb_vmid_stale) {
> >> +		/* Force vttbr to be written */
> >> +		isb();
> >> +		/* Local invalidate only for this VMID */
> >> +		write_sysreg(0, TLBIALL);
> >> +		dsb(nsh);
> >> +		vcpu->arch.tlb_vmid_stale = false;
> >> +	}
> >> +
> >
> > why not call this directly when you notice it via kvm_call_hyp as
> > opposed to adding another conditional in the critical path?
> 
> Because the cost of a hypercall is very likely to be a lot higher than
> that of testing a variable. Not to mention that at this point we're
> absolutely sure that we're going to run the guest, while the hook in
> vcpu_load is only probabilistic.
> 
Hmmm, I think for for performance workloads you care about, you will pin
VCPUs, so you'd rather take this hit in the case where your're bouncing
VCPUs all over the place, as opposed to the situation where you care
about being able to quickly exit, for example to take an interrupt for a
passthrough device.

As a comparison, the many-many debug flag checks and function calls we
currently take costs us over 150 cycles for each save/restore on both
Seattle and Mustang, so I don't think the conditionals are free.

I think doing this in the non-preemptible load_vcpu part is just fine,
and it's in line with the other tlbi stuff we do.

-Christoffer