Re: [PATCH 18/24] Exiting from L2 to L1

[Sheng Yang <sheng@linux.intel.com>]

On Monday 13 September 2010 01:21:29 Avi Kivity wrote:
>   On 09/12/2010 07:05 PM, Nadav Har'El wrote:
> >> I don't think so.  We write this field as part of guest entry (that is,
> >> after the decision re which vmcs to use, yes?), so guest entry will
> >> always follow writing this field.  Since guest entry clears the field,
> >> reading it after an exit will necessarily return 0.
> > 
> > Well, you obviously know the KVM code much better than I do, but from
> > what I saw, I thought (maybe I misunderstood) that in normal
> > (non-nested) KVM, this field only gets written on injection, not on
> > every entry, so the code relies on the fact that the processor turns off
> > the "valid" bit during exit, to avoid the same event being injected when
> > the same field value is used for another entry.
> 
> Correct.
> 
> > I can only find code which resets this field in vmx_vcpu_reset(),
> > but that doesn't get called on every entry, right? Or am I missing
> > something?
> 
> prepare_vmcs12() is called in response for a 2->1 vmexit which is first
> trapped by 0, yes?  Because it's called immediately after a vmexit,
> VM_ENTRY_INTR_INFO_FIELD is guaranteed to have been cleared by the
> processor.
> 
> There are two cases where VM_ENTRY_INTR_INFO_FIELD can potentially not
> be cleared by hardware:
> 
> 1. if we call prepare_vmcs12() between injection and entry.  This cannot
> happen AFAICT.
> 2. if the vmexit was really a failed 1->2 vmentry, and if the processor
> doesn't clear VM_ENTRY_INTR_INFO_FIELD in response to vm entry failures
> (need to check scripture)
> 
> If neither of these are valid, the code can be removed.  If only the
> second, we might make it conditional.
> 
> >> What can happen is that the contents of the field is transferred to the
> >> IDT_VECTORING_INFO field or VM_EXIT_INTR_INFO field.
> >> 
> >> (question: on a failed vmentry, is this field cleared?)
> > 
> > I don't know the answer :-)
> 
> Sheng?

According to SDM 23.7 "VM-ENTRY FAILURES DURING OR AFTER LOADING
GUEST STATE":

Although this process resembles that of a VM exit, many steps taken during a VM 
exit do not occur for these VM-entry failures:
• Most VM-exit information fields are not updated (see step 1 above).
• The valid bit in the VM-entry interruption-information field is *not* cleared.
• The guest-state area is not modified.
• No MSRs are saved into the VM-exit MSR-store area.

So VM entry failure would result in _keep_ valid bit of VM_ENTRY_INTR_INFO_FIELD.

--
regards
Yang, Sheng