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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html