Avi Kivity wrote: > 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. >> >> +/* writes trigger an event */ >> +static void >> +iosignalfd_write(struct kvm_io_device *this, gpa_t addr, int len, >> + const void *val) >> +{ >> + struct _iosignalfd *iosignalfd = (struct _iosignalfd >> *)this->private; >> + >> + eventfd_signal(iosignalfd->file, 1); >> +} >> > > I much prefer including kvm_io_device inside _iosignalfd and using > container_of() instead of ->private. But that is of course unrelated > to this patch and is not a requirement. > >> + >> +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 *iosignalfd; >> + struct file *file; >> + int ret; >> + >> + file = eventfd_fget(args->fd); >> + if (IS_ERR(file)) { >> + ret = PTR_ERR(file); >> + printk(KERN_ERR "iosignalfd: failed to get %d eventfd: %d\n", >> + args->fd, ret); >> > > drop the printk, we don't want to let users spam dmesg. > >> + return ret; >> + } >> + >> + iosignalfd = kzalloc(sizeof(*iosignalfd), GFP_KERNEL); >> + if (!iosignalfd) { >> + printk(KERN_ERR "iosignalfd: memory pressure\n"); >> > > here too. > >> + ret = kvm_io_bus_register_dev(bus, &iosignalfd->dev); >> + if (ret < 0) { >> + printk(KERN_ERR "iosignalfd: failed to register IODEV: %d\n", >> + ret); >> > > and here etc. Ack on the printk removals. > > What happens if you register to iosignalfds for the same address but > with different cookies (a very practical scenario)? This is really only supported at the iosignal interface level. Today, you can do this and the registration will succeed, but at run-time an IO-exit will stop at the first in_range() hit it finds. Therefore, you will only get service on the first/lowest registered range. I knew this was a limitation of the current io_bus, but I put the feature into iosignalfd anyway so that the user/kern interface was robust enough to support the notion should we ever need it (and can thus patch io_bus at that time). Perhaps that is short-sighted because userspace would never know its ranges weren't really registered properly. I guess its simple enough to have io_bus check all devices for a match instead of stopping on the first. Should I just make a patch to fix this, or should I fix iosignalfd to check for in_range matches and fail if it finds overlap? (We could then add a CAP_OVERLAP_IO bit in the future if we finally fix the io_bus capability). I am inclined to lean towards option 2, since its not known whether this will ever be useful, and io_bus scanning is in a hot-path. Thinking about it some more, I wonder if we should just get rid of the notion of overlap to begin with. Its a slippery slope (should we also return to userspace after scanning and matching io_bus to see if it has any overlap too?). I am not sure if it would ever be used (real hardware doesn't have multiple devices at the same address), and we can always have multiple end-points mux from one iosignalfd if we really need that. Thoughts? -Greg
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