[+cc kvm list] On Wed, Mar 19, 2014 at 7:32 PM, Ming Lei <tom.leiming@xxxxxxxxx> wrote: > On Thu, Mar 20, 2014 at 12:45 AM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote: >> On Tue, Mar 18, 2014 at 10:52 PM, Ming Lei <tom.leiming@xxxxxxxxx> wrote: >>> Hi, >>> >>> Looks Sasha fixed the problem in lkvm tool[1]. >>> >>> Sasha, looks we both saw the problem, but from technical >>> view, I am wondering if the fix is correct, because PCI spec. >>> requires that the IO/MMIO bits in COMMAND register should >>> be cleared after reset, maybe there are some potential problem >>> in lkvm pci emulation. >> >> I think I'm going to revert this patch ([2], "Ignore BAR contents when >> firmware left decoding disabled"). The main reason for that patch was >> to try for a consistent way of figuring out whether BARs are valid >> that we could use on all architectures, but I think we can do it in a >> better way. >> >> That said, this kvm change should not be necessary. We *should* be >> able to take any PCI device and initialize it from power-on state >> without any dependencies on what the BIOS left in the BARs or the >> command register. As far as I can tell, the PCI core actually worked >> fine in this case (we assigned valid addresses to the devices), but >> something else blew up. If I revert that patch, it will cover up >> whatever this other bug is, but it would be much better to figure out >> what it is and fix is. I think I figured out what the problem is. In virtio_pci__init(), we allocate some address space with pci_get_io_space_block(), save its address in vpci->mmio_addr, and hook that address space up to virtio_pci__io_mmio_callback with kvm__register_mmio(). But when we update the BAR value in pci__config_wr(), the address space mapping is never updated. I think this means that virtio-pci can't tolerate its devices being moved by the OS. In my opinion, this is a bug in linux-kvm. We've managed to avoid triggering this bug by preventing Linux from moving the BAR (either by me reverting my patch, or by Sasha's linux-kvm change [1]). But it's not very robust to assume that the OS will never change the BAR, so it's quite possible that you'll trip over this again in the future. Bjorn [1] 6478ce1416aa kvm tools: mark our PCI card as PIO and MMIO able -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
