Re: [PATCH] KVM: PPC: Convert openpic lock to raw_spinlock

From: Purcareata Bogdan
Date: Tue Feb 17 2015 - 07:28:09 EST

Ping?

On 02.02.2015 11:35, Purcareata Bogdan wrote:
Ping?

On 22.01.2015 11:39, Bogdan Purcareata wrote:
This patch enables running intensive I/O workloads, e.g. netperf, in a guest
deployed on a RT host. It also enable guests to be SMP.

The openpic spinlock becomes a sleeping mutex on a RT system. This no longer
guarantees that EPR is atomic with exception delivery. The guest VCPU thread
fails due to a BUG_ON(preemptible()) when running netperf.

In order to make the kvmppc_mpic_set_epr() call safe on RT from non-atomic
context, convert the openpic lock to a raw_spinlock. A similar approach can
be seen for x86 platforms in the following commit [1].

Here are some comparative cyclitest measurements run inside a high priority RT
guest run on a RT host. The guest has 1 VCPU and the test has been run for 15
minutes. The guest runs ~750 hackbench processes as background stress.

spinlock raw_spinlock
Min latency (us) 4 4
Avg latency (us) 15 19
Max latency (us) 70 62

Due to the introduction of the raw_spinlock, guests with a high number of VCPUs
may induce great latencies on the underlying RT Linux system (e.g. cyclictest
reports latencies of ~15ms for guests with 24 VCPUs). This can be further
aggravated by sending a lot of external interrupts to the guest. A malicious app
can abuse this scenario, causing a DoS of the host Linux. Until the KVM openpic
code is refactored to use finer lock granularity, impose a limitation on the
number of VCPUs a guest can have when running on a PREEMPT_RT_FULL system with
KVM_MPIC emulation.

Sent against v3.14-rt branch of
git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git

[1] https://lkml.org/lkml/2010/1/11/289

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@xxxxxxxxxxxxx>
[ add KVM_MAX_VCPUS limitation ]
Signed-off-by: Mihai Caraman <mihai.caraman@xxxxxxxxxxxxx>
Reviewed-by: Scott Wood <scottwood@xxxxxxxxxxxxx>
---
arch/powerpc/include/asm/kvm_host.h | 6 +++++
arch/powerpc/kvm/mpic.c | 44 ++++++++++++++++++-------------------
2 files changed, 28 insertions(+), 22 deletions(-)

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 1eaea2d..5ae38c5 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -35,8 +35,14 @@
#include <asm/page.h>
#include <asm/cacheflush.h>

+#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_KVM_MPIC)
+/* Limit the number of vcpus due to in-kernel mpic concurrency */
+#define KVM_MAX_VCPUS 4
+#define KVM_MAX_VCORES 4
+#else
#define KVM_MAX_VCPUS NR_CPUS
#define KVM_MAX_VCORES NR_CPUS
+#endif
#define KVM_USER_MEM_SLOTS 32
#define KVM_MEM_SLOTS_NUM KVM_USER_MEM_SLOTS

diff --git a/arch/powerpc/kvm/mpic.c b/arch/powerpc/kvm/mpic.c
index efbd996..b9802a3 100644
--- a/arch/powerpc/kvm/mpic.c
+++ b/arch/powerpc/kvm/mpic.c
@@ -194,7 +194,7 @@ struct openpic {
int num_mmio_regions;

gpa_t reg_base;
- spinlock_t lock;
+ raw_spinlock_t lock;

/* Behavior control */
struct fsl_mpic_info *fsl;
@@ -1105,9 +1105,9 @@ static int openpic_cpu_write_internal(void *opaque, gpa_t addr,
mpic_irq_raise(opp, dst, ILR_INTTGT_INT);
}

- spin_unlock(&opp->lock);
+ raw_spin_unlock(&opp->lock);
kvm_notify_acked_irq(opp->kvm, 0, notify_eoi);
- spin_lock(&opp->lock);
+ raw_spin_lock(&opp->lock);

break;
}
@@ -1182,12 +1182,12 @@ void kvmppc_mpic_set_epr(struct kvm_vcpu *vcpu)
int cpu = vcpu->arch.irq_cpu_id;
unsigned long flags;

- spin_lock_irqsave(&opp->lock, flags);
+ raw_spin_lock_irqsave(&opp->lock, flags);

if ((opp->gcr & opp->mpic_mode_mask) == GCR_MODE_PROXY)
kvmppc_set_epr(vcpu, openpic_iack(opp, &opp->dst[cpu], cpu));

- spin_unlock_irqrestore(&opp->lock, flags);
+ raw_spin_unlock_irqrestore(&opp->lock, flags);
}

static int openpic_cpu_read_internal(void *opaque, gpa_t addr,
@@ -1387,9 +1387,9 @@ static int kvm_mpic_read(struct kvm_io_device *this, gpa_t addr,
return -EINVAL;
}

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);
ret = kvm_mpic_read_internal(opp, addr - opp->reg_base, &u.val);
- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);

/*
* Technically only 32-bit accesses are allowed, but be nice to
@@ -1427,10 +1427,10 @@ static int kvm_mpic_write(struct kvm_io_device *this, gpa_t addr,
return -EOPNOTSUPP;
}

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);
ret = kvm_mpic_write_internal(opp, addr - opp->reg_base,
*(const u32 *)ptr);
- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);

pr_debug("%s: addr %llx ret %d val %x\n",
__func__, addr, ret, *(const u32 *)ptr);
@@ -1501,14 +1501,14 @@ static int access_reg(struct openpic *opp, gpa_t addr, u32 *val, int type)
if (addr & 3)
return -ENXIO;

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);

if (type == ATTR_SET)
ret = kvm_mpic_write_internal(opp, addr, *val);
else
ret = kvm_mpic_read_internal(opp, addr, val);

- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);

pr_debug("%s: type %d addr %llx val %x\n", __func__, type, addr, *val);

@@ -1545,9 +1545,9 @@ static int mpic_set_attr(struct kvm_device *dev, struct kvm_device_attr *attr)
if (attr32 != 0 && attr32 != 1)
return -EINVAL;

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);
openpic_set_irq(opp, attr->attr, attr32);
- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);
return 0;
}

@@ -1592,9 +1592,9 @@ static int mpic_get_attr(struct kvm_device *dev, struct kvm_device_attr *attr)
if (attr->attr > MAX_SRC)
return -EINVAL;

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);
attr32 = opp->src[attr->attr].pending;
- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);

if (put_user(attr32, (u32 __user *)(long)attr->addr))
return -EFAULT;
@@ -1670,7 +1670,7 @@ static int mpic_create(struct kvm_device *dev, u32 type)
opp->kvm = dev->kvm;
opp->dev = dev;
opp->model = type;
- spin_lock_init(&opp->lock);
+ raw_spin_lock_init(&opp->lock);

add_mmio_region(opp, &openpic_gbl_mmio);
add_mmio_region(opp, &openpic_tmr_mmio);
@@ -1743,7 +1743,7 @@ int kvmppc_mpic_connect_vcpu(struct kvm_device *dev, struct kvm_vcpu *vcpu,
if (cpu < 0 || cpu >= MAX_CPU)
return -EPERM;

- spin_lock_irq(&opp->lock);
+ raw_spin_lock_irq(&opp->lock);

if (opp->dst[cpu].vcpu) {
ret = -EEXIST;
@@ -1766,7 +1766,7 @@ int kvmppc_mpic_connect_vcpu(struct kvm_device *dev, struct kvm_vcpu *vcpu,
vcpu->arch.epr_flags |= KVMPPC_EPR_KERNEL;

out:
- spin_unlock_irq(&opp->lock);
+ raw_spin_unlock_irq(&opp->lock);
return ret;
}

@@ -1796,9 +1796,9 @@ static int mpic_set_irq(struct kvm_kernel_irq_routing_entry *e,
struct openpic *opp = kvm->arch.mpic;
unsigned long flags;

- spin_lock_irqsave(&opp->lock, flags);
+ raw_spin_lock_irqsave(&opp->lock, flags);
openpic_set_irq(opp, irq, level);
- spin_unlock_irqrestore(&opp->lock, flags);
+ raw_spin_unlock_irqrestore(&opp->lock, flags);

/* All code paths we care about don't check for the return value */
return 0;
@@ -1810,14 +1810,14 @@ int kvm_set_msi(struct kvm_kernel_irq_routing_entry *e,
struct openpic *opp = kvm->arch.mpic;
unsigned long flags;

- spin_lock_irqsave(&opp->lock, flags);
+ raw_spin_lock_irqsave(&opp->lock, flags);

/*
* XXX We ignore the target address for now, as we only support
* a single MSI bank.
*/
openpic_msi_write(kvm->arch.mpic, MSIIR_OFFSET, e->msi.data);
- spin_unlock_irqrestore(&opp->lock, flags);
+ raw_spin_unlock_irqrestore(&opp->lock, flags);

/* All code paths we care about don't check for the return value */
return 0;

--
To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/