Re: [PATCH] KVM: arm64: vgic: Fix soft lockup during VM teardown

Marc Zyngier
2023-01-18T12:34:18Z

Shanker,

Please Cc all the KVM/arm64 reviewers when sending KVM/arm64 patches.

On Wed, 18 Jan 2023 02:23:48 +0000,
Shanker Donthineni <sdonthineni@nvidia.com> wrote:
>
> Getting intermittent CPU soft lockups during the virtual machines
> teardown on a system with GICv4 features enabled. The function
> __synchronize_hardirq() has been waiting for IRQD_IRQ_INPROGRESS
> to be cleared forever as per the current implementation.
>
> CPU stuck here for a long time leads to soft lockup:
> while (irqd_irq_inprogress(&desc->irq_data))
> cpu_relax();

Is it a soft-lockup from which the system recovers? or a livelock
which leaves the system dead?

What kernel version is that? Please provide all the relevant context
that could help analysing the issue.

>
> Call trace from the lockup CPU:
> [ 87.238866] watchdog: BUG: soft lockup - CPU#37 stuck for 23s!
> [ 87.250025] CPU: 37 PID: 1031 Comm: qemu-system-aarch64
> [ 87.358397] Call trace:
> [ 87.360891] __synchronize_hardirq+0x48/0x140
> [ 87.365343] free_irq+0x138/0x424
> [ 87.368727] vgic_v4_teardown+0xa4/0xe0
> [ 87.372649] __kvm_vgic_destroy+0x18c/0x194
> [ 87.376922] kvm_vgic_destroy+0x28/0x3c
> [ 87.380839] kvm_arch_destroy_vm+0x24/0x44
> [ 87.385024] kvm_destroy_vm+0x158/0x2c4
> [ 87.388943] kvm_vm_release+0x6c/0x98
> [ 87.392681] __fput+0x70/0x220
> [ 87.395800] ____fput+0x10/0x20
> [ 87.399005] task_work_run+0xb4/0x23c
> [ 87.402746] do_exit+0x2bc/0x8a4
> [ 87.406042] do_group_exit+0x34/0xb0
> [ 87.409693] get_signal+0x878/0x8a0
> [ 87.413254] do_notify_resume+0x138/0x1530
> [ 87.417440] el0_svc+0xdc/0xf0
> [ 87.420559] el0t_64_sync_handler+0xf0/0x11c
> [ 87.424919] el0t_64_sync+0x18c/0x190
>
> The state of the IRQD_IRQ_INPROGRESS information is lost inside
> irq_domain_activate_irq() which happens before calling free_irq().
> Instrumented the code and confirmed, the IRQD state is changed from
> 0x10401400 to 0x10441600 instead of 0x10401600 causing problem.
>
> Call trace from irqd_set_activated():
> [ 78.983544] irqd_set_activated: lost IRQD_IRQ_INPROGRESS
> old=0x10401400, new=0x10441600
> [ 78.992093] CPU: 19 PID: 1511 Comm: qemu-system-aarch64
> [ 79.008461] Call trace:
> [ 79.010956] dump_backtrace.part.0+0xc8/0xe0
> [ 79.015328] show_stack+0x18/0x54
> [ 79.018713] dump_stack_lvl+0x64/0x7c
> [ 79.022459] dump_stack+0x18/0x30
> [ 79.025842] irq_domain_activate_irq+0x88/0x94
> [ 79.030385] vgic_v3_save_pending_tables+0x260/0x29c
> [ 79.035463] vgic_set_common_attr+0xac/0x23c
> [ 79.039826] vgic_v3_set_attr+0x48/0x60
> [ 79.043742] kvm_device_ioctl+0x120/0x19c
> [ 79.047840] __arm64_sys_ioctl+0x42c/0xe00
> [ 79.052027] invoke_syscall.constprop.0+0x50/0xe0
> [ 79.056835] do_el0_svc+0x58/0x180
> [ 79.060308] el0_svc+0x38/0xf0
> [ 79.063425] el0t_64_sync_handler+0xf0/0x11c
> [ 79.067785] el0t_64_sync+0x18c/0x190

Are these two traces an indication of concurrent events? Or are they
far apart?

>
> irqreturn_t handle_irq_event(struct irq_desc *desc)
> {
> irqd_set(&desc->irq_data, IRQD_IRQ_INPROGRESS);
> raw_spin_unlock(&desc->lock);
>
> ret = handle_irq_event_percpu(desc);
>
> raw_spin_lock(&desc->lock);
> irqd_clear(&desc->irq_data, IRQD_IRQ_INPROGRESS);
> }

How is that relevant to this trace? Do you see this function running
concurrently with the teardown? If it matters here, it must be a VPE
doorbell, right? But you claim that this is on a GICv4 platform, while
this would only affect GICv4.1... Or are you using GICv4.1?

>
> In this particular failed case and based on traces, the two functions
> irqd_set_activated() and handle_irq_event() are concurrently modifying
> IRQD state without both holding desc->lock. The irqd_set_activated()
> execution path is reading memory 'state_use_accessors' in between set
> & clear of IRQD_IRQ_INPROGRESS state change and writing the modified
> data after executing 'irqd_clear(desc->irq_data, IRQD_IRQ_INPROGRESS)'.
>
> To fix the lockup issue, hold desc->lock when calling functions
> irq_domain_activate_irq() and irq_domain_deactivate_irq).

For that particular issue, the main problem is that we are abusing the
interrupt startup/teardown code. The locking is only a consequence of
this.

>
> Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
> ---
> arch/arm64/kvm/vgic/vgic-v3.c | 6 ++++++
> arch/arm64/kvm/vgic/vgic-v4.c | 4 ++++
> 2 files changed, 10 insertions(+)
>
> diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
> index 2074521d4a8c..e6aa909fcbe2 100644
> --- a/arch/arm64/kvm/vgic/vgic-v3.c
> +++ b/arch/arm64/kvm/vgic/vgic-v3.c
> @@ -353,22 +353,28 @@ int vgic_v3_lpi_sync_pending_status(struct kvm *kvm, struct vgic_irq *irq)
> static void unmap_all_vpes(struct vgic_dist *dist)
> {
> struct irq_desc *desc;
> + unsigned long flags;
> int i;
>
> for (i = 0; i < dist->its_vm.nr_vpes; i++) {
> desc = irq_to_desc(dist->its_vm.vpes[i]->irq);
> + raw_spin_lock_irqsave(&desc->lock, flags);
> irq_domain_deactivate_irq(irq_desc_get_irq_data(desc));
> + raw_spin_unlock_irqrestore(&desc->lock, flags);

I guess this is the guilty one, based on your analysis, and assuming
this is a v4.1 issue, not v4.

> }
> }
>
> static void map_all_vpes(struct vgic_dist *dist)
> {
> struct irq_desc *desc;
> + unsigned long flags;
> int i;
>
> for (i = 0; i < dist->its_vm.nr_vpes; i++) {
> desc = irq_to_desc(dist->its_vm.vpes[i]->irq);
> + raw_spin_lock_irqsave(&desc->lock, flags);
> irq_domain_activate_irq(irq_desc_get_irq_data(desc), false);
> + raw_spin_unlock_irqrestore(&desc->lock, flags);
> }
> }
>
> diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
> index ad06ba6c9b00..a01b8313e82c 100644
> --- a/arch/arm64/kvm/vgic/vgic-v4.c
> +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> @@ -139,8 +139,10 @@ static void vgic_v4_enable_vsgis(struct kvm_vcpu *vcpu)
> /* Transfer the full irq state to the vPE */
> vgic_v4_sync_sgi_config(vpe, irq);
> desc = irq_to_desc(irq->host_irq);
> + raw_spin_lock(&desc->lock);
> ret = irq_domain_activate_irq(irq_desc_get_irq_data(desc),
> false);
> + raw_spin_unlock(&desc->lock);

This one looks wrong. The interrupt never fires on the host (that's
the whole point of this stuff). There isn't even a handler attached to
it. How can that result in a problem?

My take on the whole thing is that we should stop playing games with
the underlying interrupt infrastructure. How about the following hack,
which is only compile-tested. Let me know if that helps.

M.

diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index 2074521d4a8c..2624963cb95b 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -350,26 +350,23 @@ int vgic_v3_lpi_sync_pending_status(struct kvm *kvm, struct vgic_irq *irq)
* The deactivation of the doorbell interrupt will trigger the
* unmapping of the associated vPE.
*/
-static void unmap_all_vpes(struct vgic_dist *dist)
+static void unmap_all_vpes(struct kvm *kvm)
{
- struct irq_desc *desc;
+ struct vgic_dist *dist = &kvm->arch.vgic;
int i;

- for (i = 0; i < dist->its_vm.nr_vpes; i++) {
- desc = irq_to_desc(dist->its_vm.vpes[i]->irq);
- irq_domain_deactivate_irq(irq_desc_get_irq_data(desc));
- }
+ for (i = 0; i < dist->its_vm.nr_vpes; i++)
+ free_irq(dist->its_vm.vpes[i]->irq, kvm_get_vcpu(kvm, i));
}

-static void map_all_vpes(struct vgic_dist *dist)
+static void map_all_vpes(struct kvm *kvm)
{
- struct irq_desc *desc;
+ struct vgic_dist *dist = &kvm->arch.vgic;
int i;

- for (i = 0; i < dist->its_vm.nr_vpes; i++) {
- desc = irq_to_desc(dist->its_vm.vpes[i]->irq);
- irq_domain_activate_irq(irq_desc_get_irq_data(desc), false);
- }
+ for (i = 0; i < dist->its_vm.nr_vpes; i++)
+ WARN_ON(vgic_v4_request_vpe_irq(kvm_get_vcpu(kvm, i),
+ dist->its_vm.vpes[i]->irq));
}

/**
@@ -394,7 +391,7 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
* and enabling of the doorbells have already been done.
*/
if (kvm_vgic_global_state.has_gicv4_1) {
- unmap_all_vpes(dist);
+ unmap_all_vpes(kvm);
vlpi_avail = true;
}

@@ -444,7 +441,7 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)

out:
if (vlpi_avail)
- map_all_vpes(dist);
+ map_all_vpes(kvm);

return ret;
}
diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
index ad06ba6c9b00..a413718be92b 100644
--- a/arch/arm64/kvm/vgic/vgic-v4.c
+++ b/arch/arm64/kvm/vgic/vgic-v4.c
@@ -222,6 +222,11 @@ void vgic_v4_get_vlpi_state(struct vgic_irq *irq, bool *val)
*val = !!(*ptr & mask);
}

+int vgic_v4_request_vpe_irq(struct kvm_vcpu *vcpu, int irq)
+{
+ return request_irq(irq, vgic_v4_doorbell_handler, 0, "vcpu", vcpu);
+}
+
/**
* vgic_v4_init - Initialize the GICv4 data structures
* @kvm: Pointer to the VM being initialized
@@ -283,8 +288,7 @@ int vgic_v4_init(struct kvm *kvm)
irq_flags &= ~IRQ_NOAUTOEN;
irq_set_status_flags(irq, irq_flags);

- ret = request_irq(irq, vgic_v4_doorbell_handler,
- 0, "vcpu", vcpu);
+ ret = vgic_v4_request_vpe_irq(vcpu, irq);
if (ret) {
kvm_err("failed to allocate vcpu IRQ%d\n", irq);
/*
diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
index 0c8da72953f0..23e280fa0a16 100644
--- a/arch/arm64/kvm/vgic/vgic.h
+++ b/arch/arm64/kvm/vgic/vgic.h
@@ -331,5 +331,6 @@ int vgic_v4_init(struct kvm *kvm);
void vgic_v4_teardown(struct kvm *kvm);
void vgic_v4_configure_vsgis(struct kvm *kvm);
void vgic_v4_get_vlpi_state(struct vgic_irq *irq, bool *val);
+int vgic_v4_request_vpe_irq(struct kvm_vcpu *vcpu, int irq);

#endif

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