On 04/25/2010 10:00 AM, Xiao Guangrong wrote:
Two cases maybe happen in kvm_mmu_get_page() function: - one case is, the goal sp is already in cache, if the sp is unsync, we only need update it to assure this mapping is valid, but not mark it sync and not write-protect sp->gfn since it not broke unsync rule(one shadow page for a gfn) - another case is, the goal sp not existed, we need create a new sp for gfn, i.e, gfn (may)has another shadow page, to keep unsync rule, we should sync(mark sync and write-protect) gfn's unsync shadow page. After enabling multiple unsync shadows, we sync those shadow pages only when the new sp not allow to become unsync(also for the unsyc rule, the new rule is: allow all pte page become unsync)
Another interesting case is to create new shadow pages in the unsync state. That can help when the guest starts a short lived process: we can avoid write protecting its pagetables completely. Even if we do sync them, we can sync them in a batch instead of one by one, saving IPIs.
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