Re:

Jason Wang
2022-03-25T07:52:19Z

On Fri, Mar 25, 2022 at 2:31 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> Bcc:
> Subject: Re: [PATCH 3/3] virtio: harden vring IRQ
> Message-ID: <20220325021422-mutt-send-email-mst@kernel.org>
> Reply-To:
> In-Reply-To: <f7046303-7d7d-e39f-3c71-3688126cc812@redhat.com>
>
> On Fri, Mar 25, 2022 at 11:04:08AM +0800, Jason Wang wrote:
> >
> > 在 2022/3/24 下午7:03, Michael S. Tsirkin 写道:
> > > On Thu, Mar 24, 2022 at 04:40:04PM +0800, Jason Wang wrote:
> > > > This is a rework on the previous IRQ hardening that is done for
> > > > virtio-pci where several drawbacks were found and were reverted:
> > > >
> > > > 1) try to use IRQF_NO_AUTOEN which is not friendly to affinity managed IRQ
> > > > that is used by some device such as virtio-blk
> > > > 2) done only for PCI transport
> > > >
> > > > In this patch, we tries to borrow the idea from the INTX IRQ hardening
> > > > in the reverted commit 080cd7c3ac87 ("virtio-pci: harden INTX interrupts")
> > > > by introducing a global irq_soft_enabled variable for each
> > > > virtio_device. Then we can to toggle it during
> > > > virtio_reset_device()/virtio_device_ready(). A synchornize_rcu() is
> > > > used in virtio_reset_device() to synchronize with the IRQ handlers. In
> > > > the future, we may provide config_ops for the transport that doesn't
> > > > use IRQ. With this, vring_interrupt() can return check and early if
> > > > irq_soft_enabled is false. This lead to smp_load_acquire() to be used
> > > > but the cost should be acceptable.
> > > Maybe it should be but is it? Can't we use synchronize_irq instead?
> >
> >
> > Even if we allow the transport driver to synchornize through
> > synchronize_irq() we still need a check in the vring_interrupt().
> >
> > We do something like the following previously:
> >
> > if (!READ_ONCE(vp_dev->intx_soft_enabled))
> > return IRQ_NONE;
> >
> > But it looks like a bug since speculative read can be done before the check
> > where the interrupt handler can't see the uncommitted setup which is done by
> > the driver.
>
> I don't think so - if you sync after setting the value then
> you are guaranteed that any handler running afterwards
> will see the new value.

The problem is not disabled but the enable. We use smp_store_relase()
to make sure the driver commits the setup before enabling the irq. It
means the read needs to be ordered as well in vring_interrupt().

>
> Although I couldn't find anything about this in memory-barriers.txt
> which surprises me.
>
> CC Paul to help make sure I'm right.
>
>
> >
> > >
> > > > To avoid breaking legacy device which can send IRQ before DRIVER_OK, a
> > > > module parameter is introduced to enable the hardening so function
> > > > hardening is disabled by default.
> > > Which devices are these? How come they send an interrupt before there
> > > are any buffers in any queues?
> >
> >
> > I copied this from the commit log for 22b7050a024d7
> >
> > "
> >
> > This change will also benefit old hypervisors (before 2009)
> > that send interrupts without checking DRIVER_OK: previously,
> > the callback could race with driver-specific initialization.
> > "
> >
> > If this is only for config interrupt, I can remove the above log.
>
>
> This is only for config interrupt.

Ok.

>
> >
> > >
> > > > Note that the hardening is only done for vring interrupt since the
> > > > config interrupt hardening is already done in commit 22b7050a024d7
> > > > ("virtio: defer config changed notifications"). But the method that is
> > > > used by config interrupt can't be reused by the vring interrupt
> > > > handler because it uses spinlock to do the synchronization which is
> > > > expensive.
> > > >
> > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > >
> > > > ---
> > > > drivers/virtio/virtio.c | 19 +++++++++++++++++++
> > > > drivers/virtio/virtio_ring.c | 9 ++++++++-
> > > > include/linux/virtio.h | 4 ++++
> > > > include/linux/virtio_config.h | 25 +++++++++++++++++++++++++
> > > > 4 files changed, 56 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > index 8dde44ea044a..85e331efa9cc 100644
> > > > --- a/drivers/virtio/virtio.c
> > > > +++ b/drivers/virtio/virtio.c
> > > > @@ -7,6 +7,12 @@
> > > > #include <linux/of.h>
> > > > #include <uapi/linux/virtio_ids.h>
> > > > +static bool irq_hardening = false;
> > > > +
> > > > +module_param(irq_hardening, bool, 0444);
> > > > +MODULE_PARM_DESC(irq_hardening,
> > > > + "Disalbe IRQ software processing when it is not expected");
> > > > +
> > > > /* Unique numbering for virtio devices. */
> > > > static DEFINE_IDA(virtio_index_ida);
> > > > @@ -220,6 +226,15 @@ static int virtio_features_ok(struct virtio_device *dev)
> > > > * */
> > > > void virtio_reset_device(struct virtio_device *dev)
> > > > {
> > > > + /*
> > > > + * The below synchronize_rcu() guarantees that any
> > > > + * interrupt for this line arriving after
> > > > + * synchronize_rcu() has completed is guaranteed to see
> > > > + * irq_soft_enabled == false.
> > > News to me I did not know synchronize_rcu has anything to do
> > > with interrupts. Did not you intend to use synchronize_irq?
> > > I am not even 100% sure synchronize_rcu is by design a memory barrier
> > > though it's most likely is ...
> >
> >
> > According to the comment above tree RCU version of synchronize_rcu():
> >
> > """
> >
> > * RCU read-side critical sections are delimited by rcu_read_lock()
> > * and rcu_read_unlock(), and may be nested. In addition, but only in
> > * v5.0 and later, regions of code across which interrupts, preemption,
> > * or softirqs have been disabled also serve as RCU read-side critical
> > * sections. This includes hardware interrupt handlers, softirq handlers,
> > * and NMI handlers.
> > """
> >
> > So interrupt handlers are treated as read-side critical sections.
> >
> > And it has the comment for explain the barrier:
> >
> > """
> >
> > * Note that this guarantee implies further memory-ordering guarantees.
> > * On systems with more than one CPU, when synchronize_rcu() returns,
> > * each CPU is guaranteed to have executed a full memory barrier since
> > * the end of its last RCU read-side critical section whose beginning
> > * preceded the call to synchronize_rcu(). In addition, each CPU having
> > """
> >
> > So on SMP it provides a full barrier. And for UP/tiny RCU we don't need the
> > barrier, if the interrupt come after WRITE_ONCE() it will see the
> > irq_soft_enabled as false.
> >
>
> You are right. So then
> 1. I do not think we need load_acquire - why is it needed? Just
> READ_ONCE should do.

See above.

> 2. isn't synchronize_irq also doing the same thing?

Yes, but it requires a config ops since the IRQ knowledge is transport specific.

>
>
> > >
> > > > + */
> > > > + WRITE_ONCE(dev->irq_soft_enabled, false);
> > > > + synchronize_rcu();
> > > > +
> > > > dev->config->reset(dev);
> > > > }
> > > > EXPORT_SYMBOL_GPL(virtio_reset_device);
> > > Please add comment explaining where it will be enabled.
> > > Also, we *really* don't need to synch if it was already disabled,
> > > let's not add useless overhead to the boot sequence.
> >
> >
> > Ok.
> >
> >
> > >
> > >
> > > > @@ -427,6 +442,10 @@ int register_virtio_device(struct virtio_device *dev)
> > > > spin_lock_init(&dev->config_lock);
> > > > dev->config_enabled = false;
> > > > dev->config_change_pending = false;
> > > > + dev->irq_soft_check = irq_hardening;
> > > > +
> > > > + if (dev->irq_soft_check)
> > > > + dev_info(&dev->dev, "IRQ hardening is enabled\n");
> > > > /* We always start by resetting the device, in case a previous
> > > > * driver messed it up. This also tests that code path a little. */
> > > one of the points of hardening is it's also helpful for buggy
> > > devices. this flag defeats the purpose.
> >
> >
> > Do you mean:
> >
> > 1) we need something like config_enable? This seems not easy to be
> > implemented without obvious overhead, mainly the synchronize with the
> > interrupt handlers
>
> But synchronize is only on tear-down path. That is not critical for any
> users at the moment, even less than probe.

I meant if we have vq->irq_pending, we need to call vring_interrupt()
in the virtio_device_ready() and synchronize the IRQ handlers with
spinlock or others.

>
> > 2) enable this by default, so I don't object, but this may have some risk
> > for old hypervisors
>
>
> The risk if there's a driver adding buffers without setting DRIVER_OK.

Probably not, we have devices that accept random inputs from outside,
net, console, input etc. I've done a round of audits of the Qemu
codes. They look all fine since day0.

> So with this approach, how about we rename the flag "driver_ok"?
> And then add_buf can actually test it and BUG_ON if not there (at least
> in the debug build).

This looks like a hardening of the driver in the core instead of the
device. I think it can be done but in a separate series.

>
> And going down from there, how about we cache status in the
> device? Then we don't need to keep re-reading it every time,
> speeding boot up a tiny bit.

I don't fully understand here, actually spec requires status to be
read back for validation in many cases.

Thanks

>
> >
> > >
> > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > index 962f1477b1fa..0170f8c784d8 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -2144,10 +2144,17 @@ static inline bool more_used(const struct vring_virtqueue *vq)
> > > > return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> > > > }
> > > > -irqreturn_t vring_interrupt(int irq, void *_vq)
> > > > +irqreturn_t vring_interrupt(int irq, void *v)
> > > > {
> > > > + struct virtqueue *_vq = v;
> > > > + struct virtio_device *vdev = _vq->vdev;
> > > > struct vring_virtqueue *vq = to_vvq(_vq);
> > > > + if (!virtio_irq_soft_enabled(vdev)) {
> > > > + dev_warn_once(&vdev->dev, "virtio vring IRQ raised before DRIVER_OK");
> > > > + return IRQ_NONE;
> > > > + }
> > > > +
> > > > if (!more_used(vq)) {
> > > > pr_debug("virtqueue interrupt with no work for %p\n", vq);
> > > > return IRQ_NONE;
> > > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > > index 5464f398912a..957d6ad604ac 100644
> > > > --- a/include/linux/virtio.h
> > > > +++ b/include/linux/virtio.h
> > > > @@ -95,6 +95,8 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue *vq);
> > > > * @failed: saved value for VIRTIO_CONFIG_S_FAILED bit (for restore)
> > > > * @config_enabled: configuration change reporting enabled
> > > > * @config_change_pending: configuration change reported while disabled
> > > > + * @irq_soft_check: whether or not to check @irq_soft_enabled
> > > > + * @irq_soft_enabled: callbacks enabled
> > > > * @config_lock: protects configuration change reporting
> > > > * @dev: underlying device.
> > > > * @id: the device type identification (used to match it with a driver).
> > > > @@ -109,6 +111,8 @@ struct virtio_device {
> > > > bool failed;
> > > > bool config_enabled;
> > > > bool config_change_pending;
> > > > + bool irq_soft_check;
> > > > + bool irq_soft_enabled;
> > > > spinlock_t config_lock;
> > > > spinlock_t vqs_list_lock; /* Protects VQs list access */
> > > > struct device dev;
> > > > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > > > index dafdc7f48c01..9c1b61f2e525 100644
> > > > --- a/include/linux/virtio_config.h
> > > > +++ b/include/linux/virtio_config.h
> > > > @@ -174,6 +174,24 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
> > > > return __virtio_test_bit(vdev, fbit);
> > > > }
> > > > +/*
> > > > + * virtio_irq_soft_enabled: whether we can execute callbacks
> > > > + * @vdev: the device
> > > > + */
> > > > +static inline bool virtio_irq_soft_enabled(const struct virtio_device *vdev)
> > > > +{
> > > > + if (!vdev->irq_soft_check)
> > > > + return true;
> > > > +
> > > > + /*
> > > > + * Read irq_soft_enabled before reading other device specific
> > > > + * data. Paried with smp_store_relase() in
> > > paired
> >
> >
> > Will fix.
> >
> > Thanks
> >
> >
> > >
> > > > + * virtio_device_ready() and WRITE_ONCE()/synchronize_rcu() in
> > > > + * virtio_reset_device().
> > > > + */
> > > > + return smp_load_acquire(&vdev->irq_soft_enabled);
> > > > +}
> > > > +
> > > > /**
> > > > * virtio_has_dma_quirk - determine whether this device has the DMA quirk
> > > > * @vdev: the device
> > > > @@ -236,6 +254,13 @@ void virtio_device_ready(struct virtio_device *dev)
> > > > if (dev->config->enable_cbs)
> > > > dev->config->enable_cbs(dev);
> > > > + /*
> > > > + * Commit the driver setup before enabling the virtqueue
> > > > + * callbacks. Paried with smp_load_acuqire() in
> > > > + * virtio_irq_soft_enabled()
> > > > + */
> > > > + smp_store_release(&dev->irq_soft_enabled, true);
> > > > +
> > > > BUG_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> > > > dev->config->set_status(dev, status | VIRTIO_CONFIG_S_DRIVER_OK);
> > > > }
> > > > --
> > > > 2.25.1
>