Messages in this thread | | | Date | Fri, 16 Mar 2012 15:30:45 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty_log() |
| |
On 03/16/2012 02:55 PM, Takuya Yoshikawa wrote:
> On Fri, 16 Mar 2012 13:03:48 +0800 > Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote: > >> For my quickly review, mmu_lock can not protect everything, if the guest page > > Yes and ... > >> is written out of the shadow page/ept table, dirty page will be lost. > > No. > >> >> There is a example: >> >> CPU A CPU B >> guest page is written by write-emulation >> >> hold mmu-lock and see dirty-bitmap >> is not be changed, then migration is >> completed. > > We do not allow this break. >
Hmm? what can avoid this? Could you please point it out?
>> >> call mark_page_dirty() to set dirty_bit map >> >> >> Right? > > > As you pointed out, we cannot assume mutual exclusion by mmu_lock. > That is why we are using atomic bitmap operations: xchg and set_bit. > > In this sense we are at least guaranteed to get the dirty page > information in dirty_bitmap - the current one or next one. >
The problem is the guest page is written before dirty-bitmap is set, we may log the dirty page in this window like above case...
> So what we should care about is to not miss the information written in > the next bitmap at the time we actually migrate the guest. >
Actually, the way log dirty page in MMU page-table is tricky:
set dirty-bitmap
allow spte to be writeable
page can be written
That means we always set dirty-bitmap _before_ page become dirty that is the reason why your bitmap-way can work.
> Actually the userspace stops the guest at the final stage and then send the > remaining pages found in the bitmap. So the above break between write and > mark_page_dirty() cannot happen IIUC. >
Maybe i'd better firstly understand why "We do not allow this break" :)
| |