Re: [PATCH] docs: Document reserved PCI addresses for QEMU

On 4/21/20 11:01 AM, Michal Privoznik wrote: Definitely a good idea. I'm pasting in some notes I added when replying to the email thread that precipitated this document. (The device named at 01.3 is misleading - lspci says it is a "Bridge", but it's actually (according to the PIIX4 spec) an ACPI (power management) and SMBus controller. All the devices in slot 1 are hard coded into qemu - they are at that address, and can't be moved or eliminated. (I think "reserved" is a bit vague - they are actually "in use"). As for the primary video card - In the bad old days (early 2010's) slot 2 was reserved for a video card and the video card had to be at slot 2, but that hasn't been the case for a very long time. The way it works now is: if slot 2 is empty and there's a video card with no PCI address, then we'll put it at slot 2. But it's completely acceptable to put a different device at slot 2 and/or to put the video card at a different address. As a matter of fact, libvirt itself will auto-assign a different type of device to slot 2, and also doesn't complain if you add a video device and slot 2 is already taken - it will just put the video device in a different slot. (the reason for reserving slot 2 in the past was that certain *very* old versions of qemu (and maybe some current versions of weird unsupported qemu platforms) could only specify a video device with the "-vga" qemu commandline option, and that option would always put the device at slot 2 (or, rather, it would put the device at the first free slot, and the only way for us to maintain guest ABI stability across runs was to enforce that slot 2 was always either assigned to the video device, or left unused in case a video device was added later)). So the short of that - slot 2 isn't "reserved" unless you are running qemu from 10 years ago, and you can put whatever you want in slot 2 on 440fx. ^^ These three devices are in the Q35 chipset, hardcoded into qemu, and can't be moved or deleted - they are always there. Again, this isn't reserved, it just usually works out that way. You can put anything you like in slot 1, and you can put the primary video device anywhere you want. I think "recommended" is a bit misleading, as it might suggest to some people that they should actually use a USB2 controller. In reality, nobody should be using USB2 controllers since they are more complicated than USB3 and also use more CPU (although admittedly they aren't as bad as USB1). The ICH9 sound chip actually *is* to be recommended, since it is the only emulated audio device that will automatically be assigned to a slot on pcie-root, and *all* qemu sound devices are conventional PCI, so all the other audio devices will by default pull in a pcie-to-pci-bridge device, and we try to avoid that since it's considered to be "legacy". Although we'll place a dmi-to-pci-bridge at this address if someone adds one to the config, it hasn't been needed for a few years (if you really need conventional PCI slots, pcie-to-pci-bridge is a much better choice, and what libvirt chooses/adds by default when needed). So again this device will never be added by libvirt, shouldn't be added by users, and so will almost never take up this address. Even pointing out that we would put a dmi-to-pci-bridge in slot 1e *were they to add one* might mislead users into thinking that slot is otherwise unavailable, or at least that they should avoid using it. In the end, I think the devices in slot 1 on 440fx, and slot 1F on q35 should be marked as mandatory/in-use (rather than reserved), while all the other addresses (including video) should be in a list labeled as "these addresses will be used by default by the listed devices *if not otherwise used*, but aren't reserved or mandatory. Oh, and also slot 0 is reserved/in-use on both pci-root and pcie-root (for the PCI bus controller I think?).