On Fri, May 30, 2014 at 09:24:24AM -0700, Andi Kleen wrote: > > > To avoid any problems with guest pages being swapped by the host we > > > pin the pages when the PEBS buffer is setup, by intercepting > > > that MSR. > > It will avoid guest page to be swapped, but shadow paging code may still drop > > shadow PT pages that build a mapping from DS virtual address to the guest page. > > You're saying the EPT code could tear down the EPT mappings? Under memory pressure yes. mmu_shrink_scan() calls prepare_zap_oldest_mmu_page() which destroys oldest mmu pages like its name says. As far as I can tell running nested guest can also result in EPT mapping to be dropped since it will create a lot of shadow pages and this will cause make_mmu_pages_available() to destroy some shadow pages and it may choose EPT pages to destroy. CCing Marcelo to confirm/correct. > > OK that would need to be prevented too. Any suggestions how? Only high level. Mark shadow pages involved in translation we want to keep and skip them in prepare_zap_oldest_mmu_page(). > > > With EPT it is less likely to happen (but still possible IIRC depending on memory > > pressure and how much memory shadow paging code is allowed to use), without EPT > > it will happen for sure. > > Don't care about the non EPT case, this is white listed only for EPT supporting > CPUs. User may still disable EPT during module load, so pebs should be dropped from a guest's cpuid in this case. > > > There is nothing, as far as I can see, that says what will happen if the > > condition is not met. I always interpreted it as undefined behaviour so > > anything can happen including CPU dies completely. You are saying above > > on one hand that CPU cannot handle any kinds of faults during write to > > DS area, but on the other hand a guest could only crash itself. Is this > > architecturally guarantied? > > You essentially would get random page faults, and the PEBS event will > be cancelled. No hangs. Is this a guest who will get those random page faults or a host? > > It's not architecturally guaranteed, but we white list anyways so > we only care about the white listed CPUs at this point. For them > I have confirmation that it works. > > -Andi -- Gleb. -- 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
