Re: [Xen-devel] Nested virtualization off VMware vSphere 6.0 with EL6 guests crashes on Xen 4.6

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

.. snip..
>>
>> > (XEN) Failed vm entry (exit reason 0x80000021) caused by invalid guest 
>> > state (4).
>>
>> 4 means invalid VMCS link pointer - interesting.
>>
>
> Hey Jan,
>
> I hadn't been able to look at this for a quite while. A couple of folks have
> showed interest in looking at this, CC-ing them.
>

and Matt (CC-ed) had been able to debug this a bit further as well:

" I tracked this down to incorrect Xen emulation of  VMWRITE, VMPTRLD,
VMLAUNCH, and VMRESUME, in which Xen is failing the
operation if the provided address can't be mapped. A L1 VMM should be
allowed to write whatever garbage it wants into VMCS. The value may
not even be used depending on other control fields.

Xen also shouldn't be setting RFLAGS.CF (VMfailInvalid) for any
condition other than an invalid VMCS-link pointer (it was setting
RFLAGS.CF when it couldn't map the bitmap pages)."

And in retrospective it makes sense that VMWare writes garbage in
VMCS - it is probably using the binary translation part at that point.

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