nVMX regression v3.13+, bisected

[Stefan Bader <stefan.bader@canonical.com>]

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Hi,

I was looking at a bug report[1] about a regression on nested VMX that started
with kernel v3.13 (same issue still existed with v3.14-rc4). The problem shows
up when running a v3.13 kernel in L0 and then trying to launch a L2 (L1 was
either a v3.2 kernel or v3.13, so seemed to have no immediate influence). L2 is
trying to boot a iso image and hangs before the isolinux boot loader displays
anything. A preinstalled hd image fails to boot, too.

I bisected this and ended up on the following commit which, when reverted made
the launch work again:

Author: Anthoine Bourgeois <bourgeois@bertin.fr>
Date:   Wed Nov 13 11:45:37 2013 +0100

    kvm, vmx: Fix lazy FPU on nested guest

    If a nested guest does a NM fault but its CR0 doesn't contain the TS
    flag (because it was already cleared by the guest with L1 aid) then we
    have to activate FPU ourselves in L0 and then continue to L2. If TS flag
    is set then we fallback on the previous behavior, forward the fault to
    L1 if it asked for.

    Signed-off-by: Anthoine Bourgeois <bourgeois@bertin.fr>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

The condition to exit to L0 seems to be according to what the description says.
Could it be that the handling in L0 is doing something wrong?

-Stefan


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