Re: [libvirt PATCH] xenconfig: Add feature gfx_passthru

On Wed, May 13, 2020 at 01:56:52PM +0200, Artur Puzio wrote: Existing strategies: 1. libxl__is_igd_vga_passthru checks for Intel device (0x8086 vendor) with VGA class (0x030000). This should be easy to duplicate using libvirt API, like virPCIDeviceReadID(dev, "vendor") and same for "class". 2. qemu uses a different approach and checks for device being 0000:00:02.0. Again, simple to duplicate into libvirt. As we can see above, there are already two approaches, and it isn't exactly clear which should be used. In fact, I think if those two strategies won't match at some point, the domain may fail to start, because qemu refuses to start with IGD attached (according to its definition) but without the option enabled. Looking at the libxl code, the only thing that option is used for, is to add it to the qemu command line (Artur, have I missed anything?). In that case, I think ideal solution would be to patch it at the qemu level, to enable the option automatically. I believe this is exactly the case for KVM case (vfio-pci). But I think that's non-trivial approach in Xen path (if it would be easy, I think we wouldn't have this option in the first place). For example the option makes qemu choose a different PCI bridge configuration, which is I think done before checking if IGD is attached. So, while there maybe a technical reason for having this option in qmeu (instead of auto-detection), I don't really see why user needs to provide it manually. Since libxl already has a function to detect IGD, then adding autodetection there would be a better idea. I don't know what libxl maintainers would prefer, but I guess either of "not failing if gfx_passthru=1 but IGD isn't detected" or adding "gfx_passthru=auto". BTW Here is the discussion when the option was added to qemu: https://lists.nongnu.org/archive/html/qemu-devel/2015-01/msg02756.html It may be quite helpful to understand what is it about, but also why (for example I see mentions of some specific Windows drivers). -- Best Regards, Marek Marczykowski-Górecki Invisible Things Lab A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing?