> Nope. “there are no plans to address it further or fix it in an upcoming release”.
<https://bugzilla.redhat.com/show_bug.cgi?id=1408810>
I could see that being the response of an individual open-source developer working for free. But that was IBM saying that, and people pay big bucks to IBM to fix things like this.
However if someone came along and did the work upstream to fix it, I'm sure that would be accepted.
Or if a customer turned up who wanted this, that would also be implemented.
Though, reading the closing comment, this is really CLOSED-WONTFIXYET, as in no plans.
Maybe it'd be nice to introduce a WONTFIXYET. Might be useful to fossick among features abandoned that someday become feasible.
Now if your CTO golfs with an exec at IBM, you might get somewhere.
When I worked at Red Hat (virtualization-related, but not libvirt) we had a case where a customer would be running a configuration which was... unwise. We still spent a lot of time figuring out how to help them and shipped a patch that made their life easier.
The people working there are not evil, they don't intentionally not fix things, but a bug that is seen as a minor limitation and hasn't popped up in any customer case in 5+ years simply won't get fixed. There hundreds if not thousands of other bugs that are more urgent and only so many developers, QE engineers, docs people to work on this. Even if someone wrote a patch for it, it may not get merged due to how expensive Red Hats process for shipping a change is.
"If posting to Hacker News doesn't get your support query fixed, book 9 holes with the board members."
On a serious note, I deal with bug fix / feature requests regularly in Z-Stream (kernel) in an almost weekly cadence, getting the patch upstream, getting the request to backport, making an adequate test of the feature.
Source: Worked in product security for little red, now big blue.
It appears that although for some devices VM works fine but for others the VM refuses to boot (esp e100)
So the answer might be more nuanced than it seems?
For example they took ownership of X11 only so they can let it die in favor of their preferred Wayland. While Wayland is not bad, it's not covering everything.
But anyway I don't really care anymore, I'm less and less invested in the Linux ecosystem. It's too commercial now, I just stick with the BSDs <3
So, why would you need to specify this option in the first place?
No need, libvirt can pass arbitrary options to QEMU.
These days, instead of crafting a custom script to launch QEMU/KVM for PCI mapping, it's just a few clicks in virt-manager. Note that the first time you launch a VM with a mapped PCI device, the launch will often fail with an error, but it will work on a subsequent retry and thereafter.
Also, I've tinkered with lots of VMs over the past 15 years and I've NEVER had a need for more than 14 buses. Hopefully I never will.
Not sure it will work though: I need to add an option to a `pcie-root-port` command-line argument managed by libvirt.
I can try skipping creating `pcie-root-port`s by libvirt completely, and add them manually using options passthrough, but I'm not sure the rest of libvirt won't throw a fit when it finds other devices that refer to these (unknown to libvirt) PCIe slots.
I can only imagine trying to passthrough 20 nvme devices to a guest, but it seems like a very weird configuration.
On IaaS providers, you get "local scratch NVMe" presented to the guest as individual fixed-sized disks — presumably because they're being IOMMU-pass-through'ed from the host (or a JBOD direct-attached to the host.)
The sizes for these disks were standardized several generations ago, so they're at least presented to the guest as 375G slices (I'm guessing they might actually be partitions of a larger disk nowadays.) To get "decent" amounts of local scratch storage for e.g. a serverless data-warehouse instance, you need "all you can get" of these small volumes — which on at least AWS and GCP, is 24 of them (equalling ~9TB.)
And that's just one guest. The host might have several such guests.
(To be clear, neither AWS nor GCP is likely to be using libvirt anywhere in their stack. This is just to demonstrate the use-case.)
(This kind of architecture is actually "serverless", but in a possibly-arcane sense to someone who doesn't admin these sorts of systems: it's "serverless" in that your QoS isn't bounded by any "scaling factor" proportional to some number of running servers. You don't have to think about how many "servers" — or "instances" or "pods" or "containers" or whatever-else — you have running. Especially, as a customer of a "serverless" SaaS, you will only get billed for the workloads you actually run, rather than for the underlying SaaS-backend-side servers that are being temporarily reserved to run those workloads.)
Snowflake and BigQuery are examples of serverless data warehouse systems. You do a query; servers get reserved from a pool (or spun up if the pool is empty); those servers stream your at-rest data from its canonically-at-rest storage form to answer your query.
In a serverless data warehouse, as long as you still have the same server spun up and serving your queries, it'll have the data it streamed to serve your previous queries in its local disk and memory caches, making further queries on the same data "hot." The more local scratch NVMe you give these instances, the more stuff they can keep "hot" in a session to accelerate follow-on queries or looping-over-the-dataset subqueries.
Correct. I regularly use VMs with more that 14 statically configured PCI devices using QEMU with libvirt without having to resort to qemu:cmdline.
Have you got it working with PCI or PCIe? PCI devices attached to the top-level bus do not request I/O ports unless they need to, and if they do, they request only small slice.
QEMU also allows one to put 8 static PCIe devices into a single "multifunction PCIe device", so it requests 4K I/O ports per 8 devices, giving a bigger headroom. The downside, of course, is that all these 8 devices lose individual hotpluggability, and can only be added/removed en masse.
The biggest problem is hotplug slots, each taking 4K I/O ports unless told otherwise in a way libvirt does not support as I described in the article.
I have not tried to hot [un]plug devices with this configuration. It looks as though I’m likely to be disappointed if I try. Thanks for the explanation.
To make public IPs and volumes hotpluggable without a guest agent running inside every VM one has to manage them in a way guest OS will handle hotplug using regular mechanisms. For volumes it's PCIe storage hotplug, for public IPs it's PCIe network card hotplug.
If a VM is used as a Kubernetes worker, couple of dozen volumes and public IPs attached is not an unlikely situation.
Currently if you need GPUs they come with the instance itself meaning you need to boot your VM from scratch, do the work and then shut it down to relinquish the GPU.
With hot-plug you could have continuously running VMs that only attach/detach GPUs as needed, no longer taking the overhead of a full cold boot/shutdown every time.
I tried forwarding quad NVMe's and couldn't get it working until I discovered I was hitting this limitation between the existing disks and VirtIO network card.
I even did a test adding one disk at a time until the VM stopped booting.
In early computers, most device resources were fixed, especially critical stuff like keyboard and interrupt controller. Sometimes device were jumpered, but even then you’d have the choice between a few well known ranges.
There wasn’t any configuration/negotiation protocol in the early days, it was literally defined by how the wires were connected, and a few fixed logic gates. For compatibility, it had to stay that way. x86 PCs have a lot of legacy cruft.
But early boot is also where use of that legacy stuff usually ends with modern operating systems. Pretty much all these devices have been replaced by additional modern variants that are now mostly just using regular MMIO as well (I don’t think x86 I/O ports have relevant advantages, would appreciate to be told otherwise). For devices that are supposed to work on other machines than PCs (nowadays that mostly means ARM stuff), it can even get in the way, since they don’t know about this weird I/O port address space.
So modern OSes of course prefer the newer device variants (most are also decades old by now) of keyboard, interrupt controller, etc., and since those don’t tend to use I/O ports for the aforementioned reasons, modern OSes don’t use them too much in general anymore.
It's far outside the mainstream, but the x86 task state segment allows for allowing user level tasks to do i/o on specific ports, with single port granularity. You can map memory for a task only at a page level, so you could potentially allow user-space drivers finer grained access to devices. Of course, more or less nothing uses this.
> For devices that are supposed to work on other machines than PCs (nowadays that mostly means ARM stuff), it can even get in the way, since they don’t know about this weird I/O port address space.
PCI host bridges are supposed to offer a way to interact with I/O ports if it's not something natural for the CPU. Whether or not that happens regularly, I'm not really sure.
All older machines (e.g. PowerPC Macs) mapped the I/O ports to an area of the "regular" address space. They probably still do it for legacy reasons. I think only s390 got rid completely of I/O ports because they never implemented PCI, only PCIe.
A lot of hardware has migrated from using I/O ports to memory-mapped I/O, and instead of fixed I/O addresses ACPI or a similar mechanism provides the OS with the directory of memory addresses to talk to.
For example, instead of PS/2 keyboard/mouse at I/O ports 0x0060-0x0064, ACPI provides the OS with the memory address to talk to a USB controller, and the USB controller does not use I/O ports at all.
Have a look at a list of the most common I/O ports: https://wiki.osdev.org/I/O_Ports#The_list
Most of this hardware is gone. The easiest way to see them at all is to boot a VM in QEMU and specifically ask for these ancient devices to be present.
