Re: [KVM PATCH v7 2/2] KVM: add iosignalfd support

Avi Kivity
2009-06-18T11:45:12Z

On 06/16/2009 04:42 PM, 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.
>
>
> +config KVM_MAX_IOSIGNALFD_ITEMS
> + int "Maximum IOSIGNALFD items per address"
> + depends on KVM
> + default "32"
> + ---help---
> + This option influences the maximum number of fd's per PIO/MMIO
> + address that are allowed to register
> +
>

Is there a per-vm limit on iosignalfds? if not, userspace can exhaust
kernel memory in that way.

We could limit the just total number of iosignafds, it's somewhat more
natural.
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index daece36..a4b427f 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -12,3 +12,5 @@ config HAVE_KVM_EVENTFD
>
> config KVM_APIC_ARCHITECTURE
> bool
> +
> +
>

Spurious, please drop.
> +/*
> + * Design note: We create one PIO/MMIO device (iosignalfd_group) which
> + * aggregates one or more iosignalfd_items. Each item points to exactly one
> + * eventfd, and can be registered to trigger on any write to the group
> + * (wildcard), or to a write of a specific value. If more than one item is to
> + * be supported, the addr/len ranges must all be identical in the group. If a
> + * trigger value is to be supported on a particular item, the group range must
> + * be exactly the width of the trigger.
> + */
> +
> +struct _iosignalfd_item {
> + struct list_head list;
> + struct file *file;
> + unsigned char *match;
> + struct rcu_head rcu;
> +};
>

Why not u64 match?

> +static int
> +iosignalfd_is_match(struct _iosignalfd_group *group,
> + struct _iosignalfd_item *item,
> + const void *val,
> + int len)
> +{
> + if (!item->match)
> + /* wildcard is a hit */
> + return true;
> +
> + if (len != group->length)
> + /* mis-matched length is a miss */
> + return false;
>

Should check length before match (i.e. require correctly sized access).

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