On 07/26/2012 05:56 PM, Alex Williamson wrote: >> >> Let's use the same syntax as for kvm device assignment. Then we can >> >> fall back on kvm when vfio is not available. We can also have an >> >> optional parameter kernel-driver to explicitly select vfio or kvm. >> > >> > This seems confusing to me, pci-assign already has options like >> > prefer_msi, share_intx, and configfd that vfio doesn't. I'm sure vfio >> > will eventually get options that pci-assign won't have. How is a user >> > supposed to figure out what options are actually available from -device >> > pci-assign,? >> >> Read the documentation. > > And libvirt is supposed to parse the qemu-docs package matching the > installed qemu binary package to figure out what's supported? I was hoping that we could avoid any change in libvirt. > >> > Isn't this the same as asking to drop all model specific >> > devices and just use -device net,model=e1000... hey, we've been there >> > before ;) Thanks, >> >> It's not. e1000 is a guest visible feature. vfio and kvm assignment do >> exactly the same thing, as far as the guest is concerned, just using a >> different driver. This is more akin to -device virtio-net,vhost=on|off >> (where we also have a default and a fallback, which wouldn't make sense >> for model=e1000). > > I understand an agree with your desire to make this transparent from the > user perspective, but I think the place to do that abstraction is > libvirt. The qemu command line is just the final step in a process that > already needs to be aware of which backend will be used. This is not > simply a small tweak to the qemu options and now I'm using vfio. It > goes something like this: > > KVM VFIO > 1. Identify the assigned device 1. Identify the assigned device > 2. Unbind from host driver 2. Identify the iommu group for the device > 3. Bind to pci-stub 3. Evaluate all the devices for the group > 4. Launch qemu 4. Unbind all devices in the group from host drivers > 5. Bind all devices in the group to vfio-pci > 6. Launch qemu In the common case, on x86 (but I'm repeating myself), the iommu group includes just one device, yes? Could we make pci-stub an alias for the corresponding vfio steps? Though I generally dislike doing magic behind the user's back. qemu and even more the kernel are low level interfaces and should behave as regularly as possible. > > I've actually already had a report from an early adopter that did > everything under the VFIO list on the right, but but happened to be > using qemu-kvm and the -device pci-assign option and couldn't figure out > what was going on. Due to KVM's poor device ownership model, it was > more than happy to bind to a device owned by vfio-pci. Imagine the > support questions we have to ask if we support both via pci-assign; In fact we had the same experience with kvm being enabled or not. We have 'info kvm' for that. > well, what version of qemu are you using and does that default to vfio > or kvm assignment or has the distro modified it to switch the default... > VFIO offers certain advantages, for instance correctly managing the > IOMMU domain on systems like Andreas' where KVM can't manage the domain > of the bridge because it doesn't understand grouping. There are also > obvious advantages in the device ownership model. Users want to be sure > they're using these things. > > Both KVM and VFIO do strive to make the device in the guest look as much > like it does on bare metal as possible, but we don't guarantee they're > identical and we don't guarantee to match each other. So in fact, we > can expect subtle difference in how the guest sees it. Things like the > capabilities exposed, the emulation/virtualization of some of those > capabilities, eventually things like express config space support and > AER error propagation. These are all a bit more than "add vhost=on to > your virtio-net-pci options and magically your networking is faster". I see. Thanks for the explanation. -- 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
