Re: [KVM PATCH v4 2/3] kvm: make io_bus interface more robust

Avi Kivity
2009-05-27T08:54:31Z

Gregory Haskins wrote:
> Today kvm_io_bus_regsiter_dev() returns void and will internally BUG_ON if it
> fails. We want to create dynamic MMIO/PIO entries driven from userspace later
> in the series, so we need to enhance the code to be more robust with the
> following changes:
>
> 1) Add a return value to the registration function
> 2) Fix up all the callsites to check the return code, handle any
> failures, and percolate the error up to the caller.
> 3) Refactor io_bus to allow "holes" in the array so device hotplug
> can add/remove devices arbitrarily.
> 4) Add an unregister function
>
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> arch/x86/kvm/i8254.c | 34 ++++++++++++++++++++++---------
> arch/x86/kvm/i8259.c | 9 +++++++-
> include/linux/kvm_host.h | 8 +++++--
> virt/kvm/coalesced_mmio.c | 8 ++++++-
> virt/kvm/ioapic.c | 9 ++++++--
> virt/kvm/kvm_main.c | 49 +++++++++++++++++++++++++++++++++++++++------
> 6 files changed, 92 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index 584e3d3..6cf84d4 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -564,36 +564,40 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
> {
> struct kvm_pit *pit;
> struct kvm_kpit_state *pit_state;
> + int ret;
>
> pit = kzalloc(sizeof(struct kvm_pit), GFP_KERNEL);
> if (!pit)
> return NULL;
>
> pit->irq_source_id = kvm_request_irq_source_id(kvm);
> - if (pit->irq_source_id < 0) {
> - kfree(pit);
> - return NULL;
> - }
> -
> - mutex_init(&pit->pit_state.lock);
> - mutex_lock(&pit->pit_state.lock);
> - spin_lock_init(&pit->pit_state.inject_lock);
> + if (pit->irq_source_id < 0)
> + goto fail;
>
> /* Initialize PIO device */
> pit->dev.read = pit_ioport_read;
> pit->dev.write = pit_ioport_write;
> pit->dev.in_range = pit_in_range;
> pit->dev.private = pit;
> - kvm_io_bus_register_dev(&kvm->pio_bus, &pit->dev);
> + ret = kvm_io_bus_register_dev(&kvm->pio_bus, &pit->dev);
> + if (ret < 0)
> + goto fail;
>
> if (flags & KVM_PIT_SPEAKER_DUMMY) {
> pit->speaker_dev.read = speaker_ioport_read;
> pit->speaker_dev.write = speaker_ioport_write;
> pit->speaker_dev.in_range = speaker_in_range;
> pit->speaker_dev.private = pit;
> - kvm_io_bus_register_dev(&kvm->pio_bus, &pit->speaker_dev);
> + ret = kvm_io_bus_register_dev(&kvm->pio_bus,
> + &pit->speaker_dev);
> + if (ret < 0)
> + goto fail;
> }
>
> + mutex_init(&pit->pit_state.lock);
> + mutex_lock(&pit->pit_state.lock);
> + spin_lock_init(&pit->pit_state.inject_lock);
> +
>

You are registering the PIT before it is initialized. That exposes a
race. The original code also did that, but at least the pit lock was
held while this was done.

> kvm->arch.vpit = pit;
> pit->kvm = kvm;
>
> @@ -613,6 +617,16 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
> kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
>
> return pit;
> +
> +fail:
> + kvm_io_bus_unregister_dev(&kvm->pio_bus, &pit->speaker_dev);
>

There's an option now to avoid speaker_dev, so this needs to be conditional.

> + kvm_io_bus_unregister_dev(&kvm->pio_bus, &pit->dev);
> +
> + if (pit->irq_source_id >= 0)
> + kvm_free_irq_source_id(kvm, pit->irq_source_id);
> +
> + kfree(pit);
> + return NULL;
> }
>
> @@ -52,7 +52,7 @@ extern struct kmem_cache *kvm_vcpu_cache;
> * in one place.
> */
> struct kvm_io_bus {
> - int dev_count;
> + spinlock_t lock;
> #define NR_IOBUS_DEVS 6
> struct kvm_io_device *devs[NR_IOBUS_DEVS];
> };
> @@ -61,8 +61,10 @@ void kvm_io_bus_init(struct kvm_io_bus *bus);
> void kvm_io_bus_destroy(struct kvm_io_bus *bus);
> struct kvm_io_device *kvm_io_bus_find_dev(struct kvm_io_bus *bus,
> gpa_t addr, int len, int is_write);
> -void kvm_io_bus_register_dev(struct kvm_io_bus *bus,
> - struct kvm_io_device *dev);
> +int kvm_io_bus_register_dev(struct kvm_io_bus *bus,
> + struct kvm_io_device *dev);
> +int kvm_io_bus_unregister_dev(struct kvm_io_bus *bus,
> + struct kvm_io_device *dev);
>
>

unregister() should return void. There's really nothing you can do to
recover from a failure.

>
> @@ -2453,21 +2455,54 @@ struct kvm_io_device *kvm_io_bus_find_dev(struct kvm_io_bus *bus,
> {
> int i;
>
> - for (i = 0; i < bus->dev_count; i++) {
> + for (i = 0; i < NR_IOBUS_DEVS; i++) {
> struct kvm_io_device *pos = bus->devs[i];
>
> - if (pos->in_range(pos, addr, len, is_write))
> + if (pos && pos->in_range(pos, addr, len, is_write))
> return pos;
> }
>

Let's keep dev_count, and just move things around on unregister.

--
error compiling committee.c: too many arguments to function