(2011/11/14 21:39), Avi Kivity wrote:
There was a patchset from Peter Zijlstra that converted mmu notifiers to be preemptible, with that, we can convert the mmu spinlock to a mutex, I'll see what happened to it.
Interesting!
There is a third method of doing write protection, and that is by write-protecting at the higher levels of the paging hierarchy. The advantage there is that write protection is O(1) no matter how large the guest is, or the number of dirty pages. To write protect all guest memory, we just write protect the 512 PTEs at the very top, and leave the rest alone. When the guest writes to a page, we allow writes for the top-level PTE that faulted, and write-protect all the PTEs that it points to.
One important point is that the guest, not GET DIRTY LOG caller, will pay for the write protection at the timing of faults. For live migration, it may be good because we have to make the guest memory converge anyway.
We can combine it with your method by having a small bitmap (say, just 64 bits) per shadow page. Each bit represents 8 PTEs (total 512 PTEs) and is set if any of those PTEs are writeable.
Yes, there seem to be some good ways to make every case work well. Takuya -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
