On 2013-02-19 17:14, Joerg Roedel wrote:
> On Tue, Feb 19, 2013 at 11:04:01AM +0100, Jan Kiszka wrote:
>> I had a look at SVM to check how it deals with this, but I'm not sure
>> if I understand the logic correctly. SVM does:
>>
>> static int nested_svm_vmexit(struct vcpu_svm *svm)
>> {
>> ...
>> /*
>> * If we emulate a VMRUN/#VMEXIT in the same host #vmexit cycle we have
>> * to make sure that we do not lose injected events. So check event_inj
>> * here and copy it to exit_int_info if it is valid.
>> * Exit_int_info and event_inj can't be both valid because the case
>> * below only happens on a VMRUN instruction intercept which has
>> * no valid exit_int_info set.
>> */
>> if (vmcb->control.event_inj & SVM_EVTINJ_VALID) {
>> struct vmcb_control_area *nc = &nested_vmcb->control;
>>
>> nc->exit_int_info = vmcb->control.event_inj;
>> nc->exit_int_info_err = vmcb->control.event_inj_err;
>> }
>>
>> nested_svm_vmexit is only called when we leave L2 toward L1, right?
>
> Right.
>
>> So, vmcb->control.event_inj might have been set on last VMRUN emulation, and
>> if that one failed, this value shall become the nested exit_int_info. So
>> far, so good.
>
> Important fact here: This L2->L1 exit is emulated in the same real
> #vmexit cycle as the VMRUN was emulated. So what happens is:
>
> 1. VMRUN intercept from L1
> 2. We emulate the VMRUN and load L2 state into VMCB
> 3. On the way back to guest mode (to actually run the L2) we
> detect a #vmexit condition
> 4. So we roll-back by calling nested_svm_vmexit()
> 5. We enter the guest again which continues execution right
> after its VMRUN instruction.
>
> So we never actually entered L2, but for L1 it has to look like it was
> in L2 and made no progress. But when coming out of a guest event_inj is
> never valid, so without the special case above we make sure that the L1
> hypervisor re-injects the event so it is not lost.
Thanks for explaining. Already assumed this in my other mail. Makes
perfect sense now.
Need to rethink if / how this maps on VMX and if there is something
special on that side.
>
>
>> But what if that injection succeeded and we are now exiting L2 past the
>> execution of VMRUN, e.g. L1 intercepts the execution of some special
>> instruction in L2? Doesn't the nested exit_int_info now gain a stale
>> value? Or does the hardware clear the valid bit int EVENTINJ on
>> successful injection? Didn't find an indication in the spec on first
>> glance.
>
> Hardware clears event_inj. If the injection was not successful the event
> is reported in exit_int_info.
>
>> Otherwise the logic seems to be like this:
>> - EVENTINJ is set to the nested value on VMRUN emulation, and only
>> there (that's in contrast to current VMX, but it makes sense)
>> - Interrupt completion with state transfer the VCPU event queues is
>> *only* performed on L2-to-L1 exits (that's like VMX is trying to do
>> it as well)
>> - There is a special case around nested.exit_required that I didn't
>> fully get yet, nor can I say how it corresponds to logic in VMX.
>
> Which special case do you mean? There are checks in
> nested_svm_check_exception() and nested_svm_intr().
What you explained above. It wasn't clear for me at that point when
exit_required is set and what the implications are.
Jan
--
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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