On 03/16/2012 02:55 PM, Takuya Yoshikawa wrote: > On Fri, 16 Mar 2012 13:03:48 +0800 > Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> 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" :) -- 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
