Re: [PATCH] KVM: nVMX: Reset RFLAGS on VM-exit

[Jan Kiszka <jan.kiszka@web.de>]

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On 2013-03-03 20:15, Nadav Har'El wrote:
> On Sun, Mar 03, 2013, Jan Kiszka wrote about "[PATCH] KVM: nVMX: Reset RFLAGS on VM-exit":
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> Ouch, how could this work so well that far? We need to clear RFLAGS to
>> the reset value as specified by the SDM. Particularly, IF must be off
>> after VM-exit!
> 
> nested_vmx_succeed() or nested_vmx_fail*() were already clearing some of
> the fields that I understood was necessary to clear. But they did not clear
> the IF - I never realised (and didn't verify now) that this is part of the
> spec. And since L1 KVM anyways enters L2 with interrupts disabled, nested KVM
> would not see a difference.

But we didn't restore rflags on vmexit either, we reused the guest value
for L1. It was pure luck (and due to KVM's short IRQ-off phase after
vmexit) when things worked. But I bet this should fix some spurious issues.

> 
>> +	vmx_set_rflags(vcpu, 0x02);
> 
> There's a macro X86_EFLAGS_BIT1 which you can use for this 0x02.

Ah, good to know. Checked vmx_vcpu_reset and found this value.

Jan



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