Re: [KVM PATCH v7 3/3] kvm: add iofd support

Gregory Haskins
2009-05-12T19:05:32Z

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Gregory Haskins wrote:
> iofd is a mechanism to register PIO/MMIO regions to trigger an eventfd
> signal when written to. Userspace can register any arbitrary address
> with a corresponding eventfd.
>

Ugg..this patch header sucks, especially given all the talk around how
we need to do them better lately :)

I will add this text as well for future versions:

--------------

Traditional MMIO/PIO exit paths are expensive because they are done
within the same context as the VCPU thread and therefore cause a VMX/SVM
"heavy-weight" exit, a transition back to userspace, and overhead with
the qemu processing of the operation. An eventfd mechanism, on the
other hand, allows the VCPU to take a very brief lightweight exit only
long enough to trigger the eventfd_signal. This means that clients of
the eventfd (supporting both userspace or kernel end-points) can
potentially get notified much more efficiently than if we were to
register through the traditional mechanism via qemu MMIO/PIO notification.

-----------
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> include/linux/kvm.h | 12 +++++
> include/linux/kvm_host.h | 2 +
> virt/kvm/eventfd.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 13 ++++++
> 4 files changed, 134 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index dfc4bcc..99b6e45 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -292,6 +292,17 @@ struct kvm_guest_debug {
> struct kvm_guest_debug_arch arch;
> };
>
> +#define KVM_IOFD_FLAG_DEASSIGN (1 << 0)
> +#define KVM_IOFD_FLAG_PIO (1 << 1)
> +
> +struct kvm_iofd {
> + __u64 addr;
> + __u32 len;
> + __u32 fd;
> + __u32 flags;
> + __u8 pad[12];
> +};
> +
> #define KVM_TRC_SHIFT 16
> /*
> * kvm trace categories
> @@ -508,6 +519,7 @@ struct kvm_irqfd {
> #define KVM_DEASSIGN_DEV_IRQ _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
> #define KVM_ASSIGN_IRQFD _IOW(KVMIO, 0x76, struct kvm_irqfd)
> #define KVM_DEASSIGN_IRQFD _IOW(KVMIO, 0x77, __u32)
> +#define KVM_IOFD _IOW(KVMIO, 0x78, struct kvm_iofd)
>
> /*
> * ioctls for vcpu fds
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 1acc528..d53cb70 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -529,5 +529,7 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
> int kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
> int kvm_deassign_irqfd(struct kvm *kvm, int fd);
> void kvm_irqfd_release(struct kvm *kvm);
> +int kvm_iofd(struct kvm *kvm, unsigned long addr, size_t len,
> + int fd, int flags);
>
> #endif
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 71afd62..8b23317 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -21,12 +21,16 @@
> */
>
> #include <linux/kvm_host.h>
> +#include <linux/kvm.h>
> #include <linux/workqueue.h>
> #include <linux/syscalls.h>
> #include <linux/wait.h>
> #include <linux/poll.h>
> #include <linux/file.h>
> #include <linux/list.h>
> +#include <linux/eventfd.h>
> +
> +#include "iodev.h"
>
> /*
> * --------------------------------------------------------------------
> @@ -185,3 +189,106 @@ kvm_irqfd_release(struct kvm *kvm)
> list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds, list)
> irqfd_release(irqfd);
> }
> +
> +/*
> + * --------------------------------------------------------------------
> + * iofd: translate a PIO/MMIO memory write to an eventfd signal.
> + *
> + * userspace can register a PIO/MMIO address with an eventfd for recieving
> + * notification when the memory has been touched.
> + * --------------------------------------------------------------------
> + */
> +
> +struct _iofd {
> + u64 addr;
> + size_t length;
> + struct file *file;
> + struct kvm_io_device dev;
> +};
> +
> +static int
> +iofd_in_range(struct kvm_io_device *this, gpa_t addr, int len, int is_write)
> +{
> + struct _iofd *iofd = (struct _iofd *)this->private;
> +
> + return ((addr >= iofd->addr && (addr < iofd->addr + iofd->length)));
> +}
> +
> +/* writes trigger an event */
> +static void
> +iofd_write(struct kvm_io_device *this, gpa_t addr, int len, const void *val)
> +{
> + struct _iofd *iofd = (struct _iofd *)this->private;
> +
> + eventfd_signal(iofd->file, 1);
> +}
> +
> +/* reads return all zeros */
> +static void
> +iofd_read(struct kvm_io_device *this, gpa_t addr, int len, void *val)
> +{
> + memset(val, 0, len);
> +}
> +
> +static void
> +iofd_destructor(struct kvm_io_device *this)
> +{
> + struct _iofd *iofd = (struct _iofd *)this->private;
> +
> + fput(iofd->file);
> + kfree(iofd);
> +}
> +
> +static int
> +kvm_assign_iofd(struct kvm *kvm, unsigned long addr, size_t len,
> + int fd, int flags)
> +{
> + int pio = flags & KVM_IOFD_FLAG_PIO;
> + struct kvm_io_bus *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
> + struct _iofd *iofd;
> + struct file *file;
> +
> + file = eventfd_fget(fd);
> + if (IS_ERR(file))
> + return PTR_ERR(file);
> +
> + iofd = kzalloc(sizeof(*iofd), GFP_KERNEL);
> + if (!iofd) {
> + fput(file);
> + return -ENOMEM;
> + }
> +
> + iofd->dev.read = iofd_read;
> + iofd->dev.write = iofd_write;
> + iofd->dev.in_range = iofd_in_range;
> + iofd->dev.destructor = iofd_destructor;
> + iofd->dev.private = iofd;
> +
> + iofd->addr = addr;
> + iofd->length = len;
> + iofd->file = file;
> +
> + kvm_io_bus_register_dev(bus, &iofd->dev);
> +
> + printk(KERN_DEBUG "registering %s iofd at %lx of size %d\n",
> + pio ? "PIO" : "MMIO", addr, (int)len);
> +
> + return 0;
> +}
> +
> +static int
> +kvm_deassign_iofd(struct kvm *kvm, unsigned long addr, size_t len,
> + int fd, int flags)
> +{
> + /* FIXME: We need an io_bus_unregister() function */
> + return -EINVAL;
> +}
> +
> +int
> +kvm_iofd(struct kvm *kvm, unsigned long addr, size_t len, int fd, int flags)
> +{
> + if (flags & KVM_IOFD_FLAG_DEASSIGN)
> + return kvm_deassign_iofd(kvm, addr, len, fd, flags);
> +
> + return kvm_assign_iofd(kvm, addr, len, fd, flags);
> +}
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7aa9f0a..a443974 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2228,6 +2228,19 @@ static long kvm_vm_ioctl(struct file *filp,
> r = kvm_deassign_irqfd(kvm, data);
> break;
> }
> + case KVM_IOFD: {
> + struct kvm_iofd entry;
> +
> + r = -EFAULT;
> + if (copy_from_user(&entry, argp, sizeof entry))
> + goto out;
> +
> + r = kvm_iofd(kvm, entry.addr, entry.len, entry.fd,
> + entry.flags);
> + if (r)
> + goto out;
> + break;
> + }
> default:
> r = kvm_arch_vm_ioctl(filp, ioctl, arg);
> }
>
> --
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