(2011/11/29 19:09), Xiao Guangrong wrote:
Sorry, CC list is lost. :(
On 11/29/2011 06:01 PM, Xiao Guangrong wrote:
Avi Kivity<avi<at> redhat.com> writes:
On 11/16/2011 06:28 AM, Takuya Yoshikawa wrote:
(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.
I don't think there is a significant difference. The number of write
faults does not change. The amount of work done per fault does, but not
by much, thanks to the writeable bitmap.
Avi,
I think it needs more thinking if only less page need be write protected.
For example, framebuffer-based device used by Xwindow, only ~64M pages needs
to be write protected, but in your way, guest will get write page fault on all
memory? Hmm?
It has some tricks but i missed?
Do you mean write protecting slot by slot is difficult in the case of O(1)?
Takuya
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