On Sun, Apr 05, 2009 at 11:41:39AM +0300, Avi Kivity wrote: > Marcelo Tosatti wrote: >> The problem is when the page is unreachable due to a higher level path >> being unlinked. Say: >> >> level 4 -> level 3 . level 2 -> level 1 (global unsync) >> >> The dot there means level 3 is not linked to level 2, so invlpg can't >> reach the global unsync at level 1. >> >> kvm_mmu_get_page does sync pages when it finds them, so the code is >> already safe for the "linking a page which contains global ptes" case >> you mention above. >> > > The recursive resync ignores global pages (or it would hit them on cr3 > switch too). > > But we have a bigger problem, invlpg can miss even if nothing is unlinked: > > access address x through global pte > -> pte instantiated into spte > switch to cr3 where x is not mapped, or mapped differently > write to pte > -> no fault since pte is unsync > invlpg x > -> no way this can hit the spte > switch cr3 back > access address x > -> use old spte > > Here's one way to make this work: > > - add a hash of global pagetables, indexed by virtual address instead > of the pagetable's gfn > - invlpg checks this hash in addition to the recursive walk > > We'd need to make the virtual address part of sp->role to avoid needing > to link the same page multiple times in the virtual address hash. Humpf, yes. It seems its too expensive/complex to handle this, for such small gain (~= 2% on AIM7 with RHEL3 guest). Are you okay with just disabling the global pages optimization? -- 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
