Il 26/02/2014 21:27, Stefan Bader ha scritto:
> On 26.02.2014 21:25, Paolo Bonzini wrote:
>> Il 26/02/2014 20:43, Stefan Bader ha scritto:
>>> 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 <[email protected]>
>>> 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 <[email protected]>
>>> Signed-off-by: Paolo Bonzini <[email protected]>
>>>
>>> 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?
>>
>> Thanks, I'll look at it tomorrow or Friday.
>>
>> Paolo
>>
> Great thanks. And maybe it helps if I actually add the link to the bug report
> as
> I had intended... :-P
I don't have my usual test machine available, but here is a possible guess.
nested_read_cr0 is the CR0 as read by L2, but here we want to look at the
CR0 value reflecting L1's setup. This would suggest the following untested
patch:
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a06f101ef64b..0d90601a2681 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6688,7 +6688,7 @@ static bool nested_vmx_exit_handled(struct kvm_vcpu *vcpu)
else if (is_page_fault(intr_info))
return enable_ept;
else if (is_no_device(intr_info) &&
- !(nested_read_cr0(vmcs12) & X86_CR0_TS))
+ !(vmcs12->guest_cr0 & X86_CR0_TS))
return 0;
return vmcs12->exception_bitmap &
(1u << (intr_info & INTR_INFO_VECTOR_MASK));
Paolo
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