On Mon, Jan 17, 2011 at 02:18:43PM +0200, Avi Kivity wrote: > On 01/17/2011 02:18 PM, Sheng Yang wrote: > >> > + > >> > + if (copy_to_user((void __user *)(entry_base + offset), val, > >> len)) > >> > + goto out; > >> > >> Instead of copying to/from userspace (which is subject to swapin, > >> unexpected values), you could include the guest written value in a > >> kvm_run structure, along with address. Qemu-kvm would use that to > >> synchronize its copy of the table, on KVM_EXIT_MSIX_ROUTING_UPDATE exit. > > > >We want to acelerate MSI-X mask bit accessing, which won't exit to userspace > >in > >the most condition. That's the cost we want to optimize. Also it's possible > >to > >userspace to read the correct value of MMIO(but mostly userspace can't write > >to it > >in order to prevent synchronize issue). > > It's also good to have the values in just one place; using userspace > makes it easy for both the kernel and userspace to see the values > (and set them after migration, if/when we extend this to virtio).
Right, thats an advantage, but: - How can userspace ever synchronize with updates by the kernel to the MSI-X entry? - Reading/writing to the userspace area must be done carefully, values must be validated before used. - Swapping issue (minor?). -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
