Re: [KVM PATCH v9 2/2] KVM: add iosignalfd support

Michael S. Tsirkin
2009-07-07T11:21:02Z

Some comments below. Sorry, I know it's late in the series, but
previously I've been too confused with complicated locking to notice
anything else :(.

On Mon, Jul 06, 2009 at 04:33:21PM -0400, Gregory Haskins wrote:
> iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd
> signal when written to by a guest. Host userspace can register any arbitrary
> IO address with a corresponding eventfd and then pass the eventfd to a
> specific end-point of interest for handling.
>
> Normal IO requires a blocking round-trip since the operation may cause
> side-effects in the emulated model or may return data to the caller.
> Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM
> "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's
> device model synchronously before returning control back to the vcpu.
>
> However, there is a subclass of IO which acts purely as a trigger for
> other IO (such as to kick off an out-of-band DMA request, etc). For these
> patterns, the synchronous call is particularly expensive since we really
> only want to simply get our notification transmitted asychronously and
> return as quickly as possible. All the sychronous infrastructure to ensure
> proper data-dependencies are met in the normal IO case are just unecessary
> overhead for signalling. This adds additional computational load on the
> system, as well as latency to the signalling path.
>
> Therefore, we provide a mechanism for registration of an in-kernel trigger
> point that allows the VCPU to only require a very brief, lightweight
> exit just long enough to signal an eventfd. This also means that any
> clients compatible with the eventfd interface (which includes userspace
> and kernelspace equally well) can now register to be notified. The end
> result should be a more flexible and higher performance notification API
> for the backend KVM hypervisor and perhipheral components.
>
> To test this theory, we built a test-harness called "doorbell". This
> module has a function called "doorbell_ring()" which simply increments a
> counter for each time the doorbell is signaled. It supports signalling
> from either an eventfd, or an ioctl().
>
> We then wired up two paths to the doorbell: One via QEMU via a registered
> io region and through the doorbell ioctl(). The other is direct via
> iosignalfd.
>
> You can download this test harness here:
>
> ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2
>
> The measured results are as follows:
>
> qemu-mmio: 110000 iops, 9.09us rtt
> iosignalfd-mmio: 200100 iops, 5.00us rtt
> iosignalfd-pio: 367300 iops, 2.72us rtt
>
> I didn't measure qemu-pio, because I have to figure out how to register a
> PIO region with qemu's device model, and I got lazy. However, for now we
> can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO,
> and -350ns for HC, we get:
>
> qemu-pio: 153139 iops, 6.53us rtt
> iosignalfd-hc: 412585 iops, 2.37us rtt
>
> these are just for fun, for now, until I can gather more data.
>
> Here is a graph for your convenience:
>
> http://developer.novell.com/wiki/images/7/76/Iofd-chart.png
>
> The conclusion to draw is that we save about 4us by skipping the userspace
> hop.
>
> --------------------
>
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> arch/x86/kvm/x86.c | 1
> include/linux/kvm.h | 15 ++
> include/linux/kvm_host.h | 10 +-
> virt/kvm/eventfd.c | 280 ++++++++++++++++++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 11 ++
> 5 files changed, 313 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0e74d98..6e4b2f5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1204,6 +1204,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> case KVM_CAP_IRQ_INJECT_STATUS:
> case KVM_CAP_ASSIGN_DEV_IRQ:
> case KVM_CAP_IRQFD:
> + case KVM_CAP_IOSIGNALFD:
> case KVM_CAP_PIT2:
> r = 1;
> break;
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index 76c6408..236f12d 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -307,6 +307,19 @@ struct kvm_guest_debug {
> struct kvm_guest_debug_arch arch;
> };
>
> +#define KVM_IOSIGNALFD_FLAG_TRIGGER (1 << 0) /* trigger is valid */

can we rename trigger -> value?

> +#define KVM_IOSIGNALFD_FLAG_PIO (1 << 1) /* is a pio (otherwise mmio) */
> +#define KVM_IOSIGNALFD_FLAG_DEASSIGN (1 << 2)
> +
> +struct kvm_iosignalfd {
> + __u64 trigger;

for length <8, it's the 8*len least significant bits that are used, right?
That's a bit ugly ... Maybe just put an 8 byte array here instead, then
the first len bytes are valid.

> + __u64 addr; /* legal pio/mmio address */
> + __u32 len; /* 1, 2, 4, or 8 bytes */
> + __s32 fd;
> + __u32 flags;
> + __u8 pad[36];
> +};
> +
> #define KVM_TRC_SHIFT 16
> /*
> * kvm trace categories
> @@ -409,6 +422,7 @@ struct kvm_guest_debug {
> #define KVM_CAP_PIT2 33
> #endif
> #define KVM_CAP_SET_BOOT_CPU_ID 34
> +#define KVM_CAP_IOSIGNALFD 35
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -517,6 +531,7 @@ struct kvm_irqfd {
> #define KVM_IRQFD _IOW(KVMIO, 0x76, struct kvm_irqfd)
> #define KVM_CREATE_PIT2 _IOW(KVMIO, 0x77, struct kvm_pit_config)
> #define KVM_SET_BOOT_CPU_ID _IO(KVMIO, 0x78)
> +#define KVM_IOSIGNALFD _IOW(KVMIO, 0x79, struct kvm_iosignalfd)
>
> /*
> * ioctls for vcpu fds
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 306bc67..5099416 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -154,6 +154,7 @@ struct kvm {
> spinlock_t lock;
> struct list_head items;
> } irqfds;
> + struct list_head iosignalfds;
> #endif
> struct kvm_vm_stat stat;
> struct kvm_arch arch;
> @@ -532,19 +533,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
>
> #ifdef CONFIG_HAVE_KVM_EVENTFD
>
> -void kvm_irqfd_init(struct kvm *kvm);
> +void kvm_eventfd_init(struct kvm *kvm);
> int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
> void kvm_irqfd_release(struct kvm *kvm);
> +int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
>
> #else
>
> -static inline void kvm_irqfd_init(struct kvm *kvm) {}
> +static inline void kvm_eventfd_init(struct kvm *kvm) {}
> static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> {
> return -EINVAL;
> }
>
> static inline void kvm_irqfd_release(struct kvm *kvm) {}
> +static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
> +{
> + return -ENOSYS;
> +}
>
> #endif /* CONFIG_HAVE_KVM_EVENTFD */
>
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 4092b8d..c03b619 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -21,6 +21,7 @@
> */
>
> #include <linux/kvm_host.h>
> +#include <linux/kvm.h>
> #include <linux/workqueue.h>
> #include <linux/syscalls.h>
> #include <linux/wait.h>
> @@ -28,6 +29,9 @@
> #include <linux/file.h>
> #include <linux/list.h>
> #include <linux/eventfd.h>
> +#include <linux/kernel.h>
> +
> +#include "iodev.h"
>
> /*
> * --------------------------------------------------------------------
> @@ -234,10 +238,12 @@ fail:
> }
>
> void
> -kvm_irqfd_init(struct kvm *kvm)
> +kvm_eventfd_init(struct kvm *kvm)
> {
> spin_lock_init(&kvm->irqfds.lock);
> INIT_LIST_HEAD(&kvm->irqfds.items);
> +

don't need this empty line

> + INIT_LIST_HEAD(&kvm->iosignalfds);
> }
>
> /*
> @@ -327,3 +333,275 @@ static void __exit irqfd_module_exit(void)
>
> module_init(irqfd_module_init);
> module_exit(irqfd_module_exit);
> +
> +/*
> + * --------------------------------------------------------------------
> + * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
> + *
> + * userspace can register a PIO/MMIO address with an eventfd for recieving

recieving -> receiving

> + * notification when the memory has been touched.
> + * --------------------------------------------------------------------
> + */
> +
> +struct _iosignalfd {
> + struct list_head list;
> + u64 addr;
> + size_t length;

"int length" should be enough: the value is 1, 2, 4 or 8.
and put wildcard near it if you want to save some space

> + struct eventfd_ctx *eventfd;
> + u64 match;

match -> value

> + struct kvm_io_device dev;
> + int wildcard:1;

don't use bitfields

> +};
> +
> +static inline struct _iosignalfd *
> +to_iosignalfd(struct kvm_io_device *dev)
> +{
> + return container_of(dev, struct _iosignalfd, dev);
> +}
> +
> +static void
> +iosignalfd_release(struct _iosignalfd *p)
> +{
> + eventfd_ctx_put(p->eventfd);
> + list_del(&p->list);
> + kfree(p);
> +}
> +
> +static bool
> +iosignalfd_in_range(struct _iosignalfd *p, gpa_t addr, int len, const void *val)
> +{
> + u64 _val;
> +
> + if (!(addr == p->addr && len == p->length))

de-morgan's laws can help simplify this

> + /* address-range must be precise for a hit */

So there's apparently no way to specify that
you want 1,2, or 4 byte writes at address X?

> + return false;
> +
> + if (p->wildcard)
> + /* all else equal, wildcard is always a hit */
> + return true;
> +
> + /* otherwise, we have to actually compare the data */
> +
> + BUG_ON(!IS_ALIGNED((unsigned long)val, len));
> +
> + switch (len) {
> + case 1:
> + _val = *(u8 *)val;
> + break;
> + case 2:
> + _val = *(u16 *)val;
> + break;
> + case 4:
> + _val = *(u32 *)val;
> + break;
> + case 8:
> + _val = *(u64 *)val;
> + break;
> + default:
> + return false;
> + }
> +
> + return _val == p->match ? true : false;

Life be simpler if we use an 8 byte array for match
and just do memcmp here.

> +}
> +
> +/*
> + * MMIO/PIO writes trigger an event if the addr/val match
> + */

single line comment can look like this:
/* MMIO/PIO writes trigger an event if the addr/val match */

> +static int
> +iosignalfd_write(struct kvm_io_device *this, gpa_t addr, int len,
> + const void *val)
> +{
> + struct _iosignalfd *p = to_iosignalfd(this);
> +
> + if (!iosignalfd_in_range(p, addr, len, val))
> + return -EOPNOTSUPP;
> +
> + eventfd_signal(p->eventfd, 1);
> + return 0;
> +}
> +
> +/*
> + * This function is called as KVM is completely shutting down. We do not
> + * need to worry about locking just nuke anything we have as quickly as possible
> + */
> +static void
> +iosignalfd_destructor(struct kvm_io_device *this)
> +{
> + struct _iosignalfd *p = to_iosignalfd(this);
> +
> + iosignalfd_release(p);
> +}
> +
> +static const struct kvm_io_device_ops iosignalfd_ops = {
> + .write = iosignalfd_write,
> + .destructor = iosignalfd_destructor,
> +};
> +
> +static bool
> +iosignalfd_overlap(struct _iosignalfd *lhs, struct _iosignalfd *rhs)

this checks both region overlap and data collision.
better split into two helpers?

> +{
> + /*
> + * Check for completely non-overlapping regions. We can simply
> + * return "false" for non-overlapping regions and be done with
> + * it.
> + */

done with it -> ignore the value

> + if ((rhs->addr + rhs->length) <= lhs->addr)
> + return false;

rhs->addr + rhs->length <= lhs->addr
is not less clear, as precedence for simple math
follows the familiar rules from school.

> +
> + if ((lhs->addr + lhs->length) <= rhs->addr)

this math can overflow.

> + return false;

or shorter:
if (rhs->addr + rhs->length <= lhs->addr ||
lhs->addr + lhs->length <= rhs->addr)
return true;

> +
> + /*
> + * If we get here, we know there is *some* overlap, but we don't
> + * yet know how much.

how much?

> Make sure its a "precise" overlap, or

precise overlap -> address/len pairs match

> + * its rejected as incompatible
> + */

"rejected" does not seem to make sense in the context of a boolean
function

> + if (lhs->addr != rhs->addr)
> + return true;
> +
> + if (lhs->length != rhs->length)
> + return true;
> +

or shorter:
if (lhs->addr != rhs->addr || lhs->length != rhs->length)
return true;

> + /*
> + * If we get here, the request should be a precise overlap
> + * between rhs+lhs. The only thing left to check is for
> + * data-match overlap. If the data match is distinctly different
> + * we can allow the two to co-exist. Any kind of wild-card
> + * consitutes an incompatible range, so reject any wild-cards,
> + * or if the match token is identical.
> + */
> + if (lhs->wildcard || rhs->wildcard || lhs->match == rhs->match)
> + return true;
> +
> + return false;
> +}

> +
> +/* assumes kvm->slots_lock write-lock held */

it seems you only need read lock?

> +static bool
> +iosignalfd_check_collision(struct kvm *kvm, struct _iosignalfd *p)
> +{
> + struct _iosignalfd *_p;
> +
> + list_for_each_entry(_p, &kvm->iosignalfds, list)
> + if (iosignalfd_overlap(_p, p))

This looks wrong: let's assume I want one signal with length 1 and one
with length 2 at address 0. They never trigger together, so it should
be ok to have 2 such devices.

> + return true;
> +
> + return false;
> +}
> +
> +static int
> +kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
> +{
> + int pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
> + struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
> + struct _iosignalfd *p;
> + struct eventfd_ctx *eventfd;
> + int ret;
> +
> + switch (args->len) {
> + case 1:
> + case 2:
> + case 4:
> + case 8:
> + break;
> + default:
> + return -EINVAL;
> + }
> +
> + eventfd = eventfd_ctx_fdget(args->fd);
> + if (IS_ERR(eventfd))
> + return PTR_ERR(eventfd);

since this eventfd is kept around indefinitely, we should keep the
file * around as well, so that this eventfd is accounted for
properly with # of open files limit set by the admin.

> +
> + p = kzalloc(sizeof(*p), GFP_KERNEL);
> + if (!p) {
> + ret = -ENOMEM;
> + goto fail;
> + }
> +
> + INIT_LIST_HEAD(&p->list);
> + p->addr = args->addr;
> + p->length = args->len;
> + p->eventfd = eventfd;
> +
> + /*
> + * A trigger address is optional, otherwise this is a wildcard
> + */

A single line comment can look like this:
/* A trigger address is optional, otherwise this is a wildcard */

> + if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER)
> + p->match = args->trigger;

For len < 8, high bits in trigger are ignored.
it's better to check that they are 0, less confusing
if the user e.g. gets the endian-ness wrong.

> + else
> + p->wildcard = true;
> +

> + down_write(&kvm->slots_lock);
> +
> + /* Verify that there isnt a match already */

Better to put documentation to where function is declared,
not where it is used.

> + if (iosignalfd_check_collision(kvm, p)) {
> + ret = -EEXIST;
> + goto unlock_fail;
> + }
> +
> + kvm_iodevice_init(&p->dev, &iosignalfd_ops);
> +
> + ret = __kvm_io_bus_register_dev(bus, &p->dev);
> + if (ret < 0)
> + goto unlock_fail;
> +
> + list_add_tail(&p->list, &kvm->iosignalfds);
> +
> + up_write(&kvm->slots_lock);
> +
> + return 0;
> +

we probably do not need an empty line after each line of code here

> +unlock_fail:
> + up_write(&kvm->slots_lock);
> +fail:
> + /*
> + * it would have never made it to the list in the failure path, so
> + * we dont need to worry about removing it
> + */

of course: you goto fail before list_add
can just kill this comment

> + kfree(p);
> +
> + eventfd_ctx_put(eventfd);
> +
> + return ret;

we probably do not need an empty line after each line of code here

> +}
> +
> +

two empty lines

> +static int
> +kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
> +{
> + int pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
> + struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
> + struct _iosignalfd *p, *tmp;
> + struct eventfd_ctx *eventfd;
> + int ret = 0;
> +
> + eventfd = eventfd_ctx_fdget(args->fd);
> + if (IS_ERR(eventfd))
> + return PTR_ERR(eventfd);
> +
> + down_write(&kvm->slots_lock);
> +
> + list_for_each_entry_safe(p, tmp, &kvm->iosignalfds, list) {
> +

kill empty line

> + if (p->eventfd != eventfd)
> + continue;

So for deassign, you ignore all arguments besides fd? Is this
intentional? Looks strange - think of multiple addresses triggering a
single eventfd. But if so, it's better to have a different ioctl with
just the fields we need.

> +
> + __kvm_io_bus_unregister_dev(bus, &p->dev);
> + iosignalfd_release(p);

a single deassign removed multiple irqfds? Looks ugly.

> + }
> +

kill empty line

> + up_write(&kvm->slots_lock);
> +
> + eventfd_ctx_put(eventfd);
> +
> + return ret;
> +}

return error status if no device was found?

> +
> +int
> +kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
> +{
> + if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
> + return kvm_deassign_iosignalfd(kvm, args);

Better check that only known flag values are present.
Otherwise when you add more flags things just break
silently.

> +
> + return kvm_assign_iosignalfd(kvm, args);
> +}
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 11595c7..5ac381b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -979,7 +979,7 @@ static struct kvm *kvm_create_vm(void)
> spin_lock_init(&kvm->mmu_lock);
> spin_lock_init(&kvm->requests_lock);
> kvm_io_bus_init(&kvm->pio_bus);
> - kvm_irqfd_init(kvm);
> + kvm_eventfd_init(kvm);
> mutex_init(&kvm->lock);
> mutex_init(&kvm->irq_lock);
> kvm_io_bus_init(&kvm->mmio_bus);
> @@ -2271,6 +2271,15 @@ static long kvm_vm_ioctl(struct file *filp,
> r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
> break;
> }
> + case KVM_IOSIGNALFD: {
> + struct kvm_iosignalfd data;
> +
> + r = -EFAULT;

this trick is nice, it saves a line of code for the closing brace
but why waste it on an empty line above then?

> + if (copy_from_user(&data, argp, sizeof data))
> + goto out;
> + r = kvm_iosignalfd(kvm, &data);
> + break;
> + }
> #ifdef CONFIG_KVM_APIC_ARCHITECTURE
> case KVM_SET_BOOT_CPU_ID:
> r = 0;