On 2014-04-11 20:35, Bandan Das wrote: > Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes: > >> On 2014-04-11 19:26, Bandan Das wrote: >>> Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes: >>> >>>> On 2014-04-11 02:27, Bandan Das wrote: >>>>> Marcelo Tosatti <mtosatti@xxxxxxxxxx> writes: >>>>> >>>>>> On Mon, Mar 31, 2014 at 05:00:23PM -0400, Bandan Das wrote: >>>>>>> For single context invalidation, we fall through to global >>>>>>> invalidation in handle_invept() except for one case - when >>>>>>> the operand supplied by L1 is different from what we have in >>>>>>> vmcs12. However, typically hypervisors will only call invept >>>>>>> for the currently loaded eptp, so the condition will >>>>>>> never be true. >>>>>>> >>>>>>> Signed-off-by: Bandan Das <bsd@xxxxxxxxxx> >>>>>> >>>>>> Bandan, >>>>>> >>>>>> Why not fix INVEPT single-context rather than removing it entirely? >>>>>> >>>>>> "Single-context. If the INVEPT type is 1, the logical processor >>>>>> invalidates all guest-physical mappings and combined mappings associated >>>>>> with the EP4TA specified in the INVEPT descriptor. Combined mappings for >>>>>> that EP4TA are invalidated for all VPIDs and all PCIDs. (The instruction >>>>>> may invalidate mappings associated with other EP4TAs.)" >>>>>> >>>>>> So just removing the "if (EPTP != CURRENT.EPTP) BREAK" should be enough. >>>>> >>>>> The single context invalidation in handle_invept() doesn't do >>>>> anything different. It just falls down to the global case. >>>>> And the invept code in Xen and KVM both seemed to fall back >>>>> to global invalidation if support for single context wasn't found. >>>>> So, it was proposed not to advertise it at all. >>>>> >>>>> But rethinking this again, I agree with you. If there's a hypervisor >>>>> with a single context invept implmentation that does not fallback, >>>>> this will unfortunately not work. Jan, do you agree with this ? >>>> >>>> A hypervisor that doesn't properly check the HW caps is just broken. And >>>> one that mandates single context invalidation support is silly. >>> >>> Well, but we could make life a little bit easier for the unfortunate user >>> using the broken hypervisor :) And advertising single context inavalidation >>> doesn't really seem to have any downsides. >> >> Ok, let's try it this way: single-context invalidation is inherently >> tied to VPID support (that's how you address a context). However, KVM >> does not expose VPID to its guest. So this discussion is mood: no >> hypervisor will make use of this feature as it has no means to fill in >> the required parameter. > > I thought (from the spec) invept single context invalidation > takes the EP4TA as the second argument. invvpid single context > however takes the VPID as its descriptor. Oops, invept/invvpid mess-up while re-reading the spec - sorry. > > The Xen L1 hypervisor was actually calling single context invept > multiple times. That's how I hit this bug. ...and it's no longer doing it now, I suppose. The question remains, which hypervisor we want to cater with a "single-context-that-is-current-context" invalidation (that is my understanding of Marcelo's proposal). On the other hand, if some hypervisor actually uses invept to invalidate a non-current mapping, we would regress compared to not exposing single context invept. Hope I got this conclusion right. ;) Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux -- 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
