On 03/21/2014 04:07 PM, Bjorn Helgaas wrote:
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.
The purpose of KVM tool is to implement as much as possible of the KVM interface and the virtio spec so that we'll have a good development/testing environment with a very simple to understand codebase. The issue you've mentioned is the "evil" side of the KVM tool. It never tried (or claimed) to implement anything close to legacy hardware interfaces. This means, for example, that it doesn't run any BIOS, there's very lacking APIC support and the kernel is just injected into the virtual RAM and gets run from there. It also means that we went into the PCI spec deep enough to get the code to work with the kernel. The only reason we implemented MSI interrupts for example is because they provide improved performance with KVM, not because we were trying to get a complete implementation of the PCI spec. So yes, the PCI implementation in the KVM tool is lacking and what we have there might be broken by making the kernel conform more closely to the spec, but we are always happy to adapt and improve our code to work with any changes in the kernel. To sum it up, If you'll end up adding a change to the kernel that is valid according to the spec but breaks the KVM tool we'll just go ahead and fix the tool. You really don't need to worry about breaking it. Thanks, Sasha -- 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
