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.