Re: Nested virtualization instabilities

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 16/12/2013 11:05, Etzion Bar-Noy wrote:
> Hi.
>
> I am using the unstable build-from-source of Xen community. It reports
> as 4.4, and was pulled from Git few days ago. The problem I will
> describe below appears on version 4.3.1, compiled from source as well.
> In both cases - I used spec file to create my own RPMs, because it's
> the right method of doing it.
>
> Problem: When VM running under HVM which is a hypervisor (XenServer
> 6.2 or VMware ESXi5.5) attempts to start a (HVM) virtual machine, the
> following happens, 100% reproducible on this server.
> XenServer: domain crash. XenServer restarts itself. It doesn't happen
> for PV on XenServer VM
> VMware: An "unknown failure", which leads me to think that the nested
> virtualization properties are not forwarded correctly to DomU
>
>

<snip>

> (XEN) <vm_launch_fail> error code 7
> (XEN) domain_crash_sync called from vmcs.c:1293
> (XEN) Domain 8 (vcpu#0) crashed on cpu#4:
> (XEN) ----[ Xen-4.4-unstable  x86_64  debug=y  Not tainted ]----
> (XEN) CPU:    4
> (XEN) RIP:    0000:[<0000000000000000>]
> (XEN) RFLAGS: 0000000000000002   CONTEXT: hvm guest
> (XEN) rax: 0000000000000000   rbx: ffff830824f98000   rcx:
> ffff830824f9ff80
> (XEN) rdx: ffff82d0801d0cf9   rsi: 0000000000000000   rdi:
> ffff82d0801de7fc
> (XEN) rbp: ffff82d080105ab1   rsp: 0000000000000000   r8: 
> 0000000000000004
> (XEN) r9:  ffff82d080105b20   r10: ffff830824f9ff70   r11:
> 0000000000000000
> (XEN) r12: ffff830824f9ff50   r13: ffff830654f9e000   r14:
> ffff830824f9ff30
> (XEN) r15: ffff82d080187e43   cr0: 0000000000000039   cr4:
> 0000000000002050
> (XEN) cr3: 00000000feffa000   cr2: 0000000000000000
> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: 0000

This is the problem.

Error code 7 is defined as "VM entry with invalid control field(s)"

At a guess, I would think that the L1 hypervisor is probing the control
fields (as that seems to be the only way of detecting whether a feature
is available or not), and this error might need bouncing back to the VM.

Otherwise, the L1 hypervisor has decided it can use a particular
control, and Xen has insufficient validation if the L1 state.

Either way, it would be useful to know if there is a way for the
hardware to identify which control field it has an issue with...

~Andrew

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