On 03/05/2017 16:50, Xiao Guangrong wrote: > Furthermore, userspace has no knowledge about if PML is enable (it > can be required from sysfs, but it is a good way in QEMU), so it is > difficult for the usespace to know when to use write-protect-all. > Maybe we can make KVM_CAP_X86_WRITE_PROTECT_ALL_MEM return false if > PML is enabled?
Yes, that's a good idea. Though it's a pity that, with PML, setting the dirty bit will still do the massive walk of the rmap. At least with reset_dirty_pages it's done a little bit at a time. >> Also, I wonder how the alternative write protection mechanism would >> affect performance of the dirty page ring buffer patches. You would do >> the write protection of all memory at the end of >> kvm_vm_ioctl_reset_dirty_pages. You wouldn't even need a separate >> ioctl, which is nice. On the other hand, checkpoints would be more >> frequent and most pages would be write-protected, so it would be more >> expensive to rebuild the shadow page tables... > > Yup, write-protect-all can improve reset_dirty_pages indeed, i will > apply your idea after reset_dirty_pages is merged. > > However, we still prefer to have a separate ioctl for write-protect-all > which cooperates with KVM_GET_DIRTY_LOG to improve live migration that > should not always depend on checkpoint. Ok, I plan to merge the dirty ring pages early in 4.13 development. Paolo
