Re: [PATCH v3 04/10] KVM: arm/arm64: use vcpu request in kvm_arm_halt_vcpu

Christoffer Dall
2017-05-16T10:06:33Z

On Tue, May 16, 2017 at 04:17:33AM +0200, Andrew Jones wrote:
> On Mon, May 15, 2017 at 01:14:42PM +0200, Christoffer Dall wrote:
> > On Tue, May 09, 2017 at 07:02:51PM +0200, Andrew Jones wrote:
> > > On Sat, May 06, 2017 at 08:08:09PM +0200, Christoffer Dall wrote:
> > > > On Wed, May 03, 2017 at 06:06:29PM +0200, Andrew Jones wrote:
> > > > > VCPU halting/resuming is partially implemented with VCPU requests.
> > > > > When kvm_arm_halt_guest() is called all VCPUs get the EXIT request,
> > > > > telling them to exit guest mode and look at the state of 'pause',
> > > > > which will be true, telling them to sleep. As ARM's VCPU RUN
> > > > > implements the memory barrier pattern described in "Ensuring Requests
> > > > > Are Seen" of Documentation/virtual/kvm/vcpu-requests.rst, there's
> > > > > no way for a VCPU halted by kvm_arm_halt_guest() to miss the pause
> > > > > state change. However, before this patch, a single VCPU halted with
> > > > > kvm_arm_halt_vcpu() did not get a request, opening a tiny race window.
> > > > > This patch adds the request, closing the race window and also allowing
> > > > > us to remove the final check of pause in VCPU RUN, as the final check
> > > > > for requests is sufficient.
> > > > >
> > > > > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > > > >
> > > > > ---
> > > > >
> > > > > I have two questions about the halting/resuming.
> > > > >
> > > > > Question 1:
> > > > >
> > > > > Do we even need kvm_arm_halt_vcpu()/kvm_arm_resume_vcpu()? It should
> > > > > only be necessary if one VCPU can activate or inactivate the private
> > > > > IRQs of another VCPU, right? That doesn't seem like something that
> > > > > should be possible, but I'm GIC-illiterate...
> > > >
> > > > True, it shouldn't be possible. I wonder if we were thinking of
> > > > userspace access to the CPU-specific data, but we already ensure that no
> > > > VCPUs are running at that time, so I don't think it should be necessary.
> > > >
> > > > >
> > > > > Question 2:
> > > > >
> > > > > It's not clear to me if we have another problem with halting/resuming
> > > > > or not. If it's possible for VCPU1 and VCPU2 to race in
> > > > > vgic_mmio_write_s/cactive(), then the following scenario could occur,
> > > > > leading to VCPU3 being in guest mode when it should not be. Does the
> > > > > hardware prohibit more than one VCPU entering trap handlers that lead
> > > > > to these functions at the same time? If not, then I guess pause needs
> > > > > to be a counter instead of a boolean.
> > > > >
> > > > > VCPU1 VCPU2 VCPU3
> > > > > ----- ----- -----
> > > > > VCPU3->pause = true;
> > > > > halt(VCPU3);
> > > > > if (pause)
> > > > > sleep();
> > > > > VCPU3->pause = true;
> > > > > halt(VCPU3);
> > > > > VCPU3->pause = false;
> > > > > resume(VCPU3);
> > > > > ...wake up...
> > > > > if (!pause)
> > > > > Enter guest mode. Bad!
> > > > > VCPU3->pause = false;
> > > > > resume(VCPU3);
> > > > >
> > > > > (Yes, the "Bad!" is there to both identify something we don't want
> > > > > occurring and to make fun of Trump's tweeting style.)
> > > >
> > > > I think it's bad, and it might be even worse, because it could lead to a
> > > > CPU looping forever in the host kernel, since there's no guarantee to
> > > > exit from the VM in the other VCPU thread.
> > > >
> > > > But I think simply taking the kvm->lock mutex to serialize the mmio
> > > > active change operations should be sufficient.
> > > >
> > > > If we agree on this I can send a patch with your reported by that fixes
> > > > that issue, which gets rid of kvm_arm_halt_vcpu and requires you to
> > > > modify your first patch to clear the KVM_REQ_VCPU_EXIT flag for each
> > > > vcpu in kvm_arm_halt_guest instead and you can fold the remaining change
> > > > from this patch into a patch that completely gets rid of the pause flag.
> > >
> > > Yup, seems reasonable to me to lock the kvm mutex on a stop the guest type
> > > action.
> > >
> > > >
> > > > See untested patch draft at the end of this mail.
> > > >
> > > > Thanks,
> > > > -Christoffer
> > > >
> > > > > ---
> > > > > arch/arm/kvm/arm.c | 3 ++-
> > > > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> > > > > index 47f6c7fdca96..9174ed13135a 100644
> > > > > --- a/arch/arm/kvm/arm.c
> > > > > +++ b/arch/arm/kvm/arm.c
> > > > > @@ -545,6 +545,7 @@ void kvm_arm_halt_guest(struct kvm *kvm)
> > > > > void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
> > > > > {
> > > > > vcpu->arch.pause = true;
> > > > > + kvm_make_request(KVM_REQ_VCPU_EXIT, vcpu);
> > > > > kvm_vcpu_kick(vcpu);
> > > > > }
> > > > >
> > > > > @@ -664,7 +665,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> > > > >
> > > > > if (ret <= 0 || need_new_vmid_gen(vcpu->kvm) ||
> > > > > kvm_request_pending(vcpu) ||
> > > > > - vcpu->arch.power_off || vcpu->arch.pause) {
> > > > > + vcpu->arch.power_off) {
> > > > > vcpu->mode = OUTSIDE_GUEST_MODE;
> > > > > local_irq_enable();
> > > > > kvm_pmu_sync_hwstate(vcpu);
> > > > > --
> > > > > 2.9.3
> > > > >
> > > >
> > > >
> > > > Untested draft patch:
> > > >
> > > > diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> > > > index d488b88..b77a3af 100644
> > > > --- a/arch/arm/include/asm/kvm_host.h
> > > > +++ b/arch/arm/include/asm/kvm_host.h
> > > > @@ -234,8 +234,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> > > > struct kvm_vcpu __percpu **kvm_get_running_vcpus(void);
> > > > void kvm_arm_halt_guest(struct kvm *kvm);
> > > > void kvm_arm_resume_guest(struct kvm *kvm);
> > > > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> > > > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> > > >
> > > > int kvm_arm_copy_coproc_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
> > > > unsigned long kvm_arm_num_coproc_regs(struct kvm_vcpu *vcpu);
> > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > > index 578df18..7a38d5a 100644
> > > > --- a/arch/arm64/include/asm/kvm_host.h
> > > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > > @@ -334,8 +334,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> > > > struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
> > > > void kvm_arm_halt_guest(struct kvm *kvm);
> > > > void kvm_arm_resume_guest(struct kvm *kvm);
> > > > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> > > > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> > > >
> > > > u64 __kvm_call_hyp(void *hypfn, ...);
> > > > #define kvm_call_hyp(f, ...) __kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__)
> > > > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > > > index 7941699..932788a 100644
> > > > --- a/virt/kvm/arm/arm.c
> > > > +++ b/virt/kvm/arm/arm.c
> > > > @@ -542,27 +542,15 @@ void kvm_arm_halt_guest(struct kvm *kvm)
> > > > kvm_make_all_cpus_request(kvm, KVM_REQ_VCPU_EXIT);
> > > > }
> > > >
> > > > -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
> > > > -{
> > > > - vcpu->arch.pause = true;
> > > > - kvm_vcpu_kick(vcpu);
> > > > -}
> > > > -
> > > > -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu)
> > > > -{
> > > > - struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> > > > -
> > > > - vcpu->arch.pause = false;
> > > > - swake_up(wq);
> > > > -}
> > > > -
> > > > void kvm_arm_resume_guest(struct kvm *kvm)
> > > > {
> > > > int i;
> > > > struct kvm_vcpu *vcpu;
> > > >
> > > > - kvm_for_each_vcpu(i, vcpu, kvm)
> > > > - kvm_arm_resume_vcpu(vcpu);
> > > > + kvm_for_each_vcpu(i, vcpu, kvm) {
> > > > + vcpu->arch.pause = false;
> > > > + swake_up(kvm_arch_vcpu_wq(vcpu));
> > > > + }
> > > > }
> > > >
> > > > static void vcpu_sleep(struct kvm_vcpu *vcpu)
> > > > diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> > > > index 2a5db13..c143add 100644
> > > > --- a/virt/kvm/arm/vgic/vgic-mmio.c
> > > > +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> > > > @@ -231,23 +231,21 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
> > > > * be migrated while we don't hold the IRQ locks and we don't want to be
> > > > * chasing moving targets.
> > > > *
> > > > - * For private interrupts, we only have to make sure the single and only VCPU
> > > > - * that can potentially queue the IRQ is stopped.
> > > > + * For private interrupts we don't have to do anything because userspace
> > > > + * accesses to the VGIC state already require all VCPUs to be stopped, and
> > > > + * only the VCPU itself can modify its private interrupts active state, which
> > > > + * guarantees that the VCPU is not running.
> > > > */
> > > > static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
> > > > {
> > > > - if (intid < VGIC_NR_PRIVATE_IRQS)
> > > > - kvm_arm_halt_vcpu(vcpu);
> > > > - else
> > > > + if (intid > VGIC_NR_PRIVATE_IRQS)
> > > > kvm_arm_halt_guest(vcpu->kvm);
> > > > }
> > > >
> > > > /* See vgic_change_active_prepare */
> > > > static void vgic_change_active_finish(struct kvm_vcpu *vcpu, u32 intid)
> > > > {
> > > > - if (intid < VGIC_NR_PRIVATE_IRQS)
> > > > - kvm_arm_resume_vcpu(vcpu);
> > > > - else
> > > > + if (intid > VGIC_NR_PRIVATE_IRQS)
> > > > kvm_arm_resume_guest(vcpu->kvm);
> > > > }
> > > >
> > > > @@ -258,6 +256,7 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
> > > > u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
> > > > int i;
> > > >
> > > > + mutex_lock(&vcpu->kvm->lock);
> > > > vgic_change_active_prepare(vcpu, intid);
> > > > for_each_set_bit(i, &val, len * 8) {
> > > > struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
> > > > @@ -265,6 +264,7 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
> > > > vgic_put_irq(vcpu->kvm, irq);
> > > > }
> > > > vgic_change_active_finish(vcpu, intid);
> > > > + mutex_unlock(&vcpu->kvm->lock);
> > > > }
> > > >
> > > > void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
> > > > @@ -274,6 +274,7 @@ void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
> > > > u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
> > > > int i;
> > > >
> > > > + mutex_lock(&vcpu->kvm->lock);
> > > > vgic_change_active_prepare(vcpu, intid);
> > > > for_each_set_bit(i, &val, len * 8) {
> > > > struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
> > > > @@ -281,6 +282,7 @@ void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
> > > > vgic_put_irq(vcpu->kvm, irq);
> > > > }
> > > > vgic_change_active_finish(vcpu, intid);
> > > > + mutex_unlock(&vcpu->kvm->lock);
> > > > }
> > > >
> > > > unsigned long vgic_mmio_read_priority(struct kvm_vcpu *vcpu,
> > >
> > > Looks good to me. How about adding kvm->lock to the locking order comment
> > > at the top of virt/kvm/arm/vgic/vgic.c too. With that, you can add my R-b
> > > on the posting.
> > >
> > > I'll rebase this series on your posting.
> > >
> >
> > FYI, this patch is now in kvmarm/queue.
>
> Just rebased the vcpu request series on it and tested. Bad news. This
> patch immediately hangs the guest for me. Letting it sit for a couple
> minutes gets the logs
>
> [ 243.691000] INFO: task qemu-kvm:1710 blocked for more than 120 seconds.
> [ 243.697591] Not tainted 4.12.0-rc1+ #3
> [ 243.701860] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 243.709653] qemu-kvm D 0 1710 1 0x00000200
> [ 243.715132] Call trace:
> [ 243.717575] [<ffff0000080857d8>] __switch_to+0x64/0x70
> [ 243.722707] [<ffff0000087b1dd4>] __schedule+0x31c/0x854
> [ 243.727909] [<ffff0000087b2340>] schedule+0x34/0x8c
> [ 243.732778] [<ffff0000087b26f8>] schedule_preempt_disabled+0x14/0x1c
> [ 243.739105] [<ffff0000087b3730>] __mutex_lock.isra.8+0x170/0x49c
> [ 243.745098] [<ffff0000087b3a80>] __mutex_lock_slowpath+0x24/0x30
> [ 243.751092] [<ffff0000087b3acc>] mutex_lock+0x40/0x4c
> [ 243.756123] [<ffff0000080baa2c>] vgic_mmio_write_cactive+0x40/0x14c
> [ 243.762370] [<ffff0000080bb694>] vgic_uaccess+0xd0/0x104
> [ 243.767662] [<ffff0000080bc100>] vgic_v2_dist_uaccess+0x70/0x94
> [ 243.773568] [<ffff0000080bdc48>] vgic_v2_attr_regs_access.isra.6+0x108/0x110
> [ 243.780587] [<ffff0000080bddec>] vgic_v2_set_attr+0xc4/0xd4
> [ 243.786148] [<ffff0000080a1028>] kvm_device_ioctl_attr+0x7c/0xc8
> [ 243.792143] [<ffff0000080a10f8>] kvm_device_ioctl+0x84/0xd4
> [ 243.797692] [<ffff00000829025c>] do_vfs_ioctl+0xcc/0x7b4
> [ 243.802988] [<ffff0000082909d4>] SyS_ioctl+0x90/0xa4
> [ 243.807933] [<ffff0000080834a0>] __sys_trace_return+0x0/0x4
>
> I don't have kdump or support for live crash setup right now to see what
> else is holding the lock. Unfortunately I don't have time to set it up
> either, as I'll be out of the office from now through the rest of the
> week.
>
> I'll still post v4 of the vcpu request series now. I've smoke tested
> it with the vgic_mmio_write_cactive/sactive locking removed.
>
Duh, we were already holding the mutex when coming from the uaccess
patch, and my testing was utterly flawed because I had forgotten that I
had configured my grub boot default for a specific kernel, and thus I
didn't actually test this patch when I thought I did.

I've sent a small v2 series for that patch and removed it from
kvmarm/queue.

Thanks for the bug report.

-Christoffer