Re: [libvirt] [PATCH 0/6] RFC: qemu: virtio-{non-}transitional support

On Sun, 2019-01-13 at 18:12 -0500, Cole Robinson wrote: [...] > The main RFC bits here are:

> * The PCI address handling. I just mapped virtio-non-transitional > to imply plain PCI addressing. I think that's all we need but > I'm not positive so I'd appreciate a review of that approach. I don't think that's right. Let me fish up a message I wrote a while ago summing up interactions between VirtIO devices and PCI (Express) slots: http://lists.nongnu.org/archive/html/qemu-devel/2018-11/msg03133.html Basically VirtIO 0.9 requires IO space to be available, and 1.0 did away with that requirement because PCI Express, unlike conventional PCI, allows devices *not* to have IO space. So transitional devices, which must work with both 0.9 and 1.0, can depend on IO space being available and as such will only work when plugged into conventional PCI slots, whereas non-transitional devices don't need IO space and can thus be plugged into either conventional PCI and PCI Express slots. Ultimately, then, transitional (rather than non-transitional) devices are the ones that must be forced into conventional PCI slots.