On Sun, Aug 25, 2013 at 3:37 PM, Abel Gordon <ABELG@xxxxxxxxxx> wrote:
>
>
>> From: Jan Kiszka <jan.kiszka@xxxxxx>
>> To: "李春奇 <Arthur Chunqi Li>" <yzt356@xxxxxxxxx>,
>> Cc: kvm@xxxxxxxxxxxxxxx, gleb@xxxxxxxxxx, pbonzini@xxxxxxxxxx
>> Date: 25/08/2013 09:44 AM
>> Subject: Re: [PATCH] KVM: nVMX: Fully support of nested VMX preemption
> timer
>> Sent by: kvm-owner@xxxxxxxxxxxxxxx
>>
>> On 2013-08-24 20:44, root wrote:
>> > This patch contains the following two changes:
>> > 1. Fix the bug in nested preemption timer support. If vmexit L2->L0
>> > with some reasons not emulated by L1, preemption timer value should
>> > be save in such exits.
>> > 2. Add support of "Save VMX-preemption timer value" VM-Exit controls
>> > to nVMX.
>> >
>> > With this patch, nested VMX preemption timer features are fully
>> > supported.
>> >
>> > Signed-off-by: Arthur Chunqi Li <yzt356@xxxxxxxxx>
>> > ---
>
>> >
>> > @@ -7578,9 +7579,14 @@ static void prepare_vmcs02(struct kvm_vcpu
>> *vcpu, struct vmcs12 *vmcs12)
>> > (vmcs_config.pin_based_exec_ctrl |
>> > vmcs12->pin_based_vm_exec_control));
>> >
>> > - if (vmcs12->pin_based_vm_exec_control &
> PIN_BASED_VMX_PREEMPTION_TIMER)
>> > - vmcs_write32(VMX_PREEMPTION_TIMER_VALUE,
>> > - vmcs12->vmx_preemption_timer_value);
>> > + if (vmcs12->pin_based_vm_exec_control &
>> PIN_BASED_VMX_PREEMPTION_TIMER) {
>> > + if (vmcs12->vm_exit_controls &
> VM_EXIT_SAVE_VMX_PREEMPTION_TIMER)
>> > + vmcs12->vmx_preemption_timer_value =
>> > + vmcs_read32(VMX_PREEMPTION_TIMER_VALUE);
>> > + else
>> > + vmcs_write32(VMX_PREEMPTION_TIMER_VALUE,
>> > + vmcs12->vmx_preemption_timer_value);
>> > + }
>>
>> This is not correct. We still need to set the vmcs to
>> vmx_preemption_timer_value. The difference is that, on exit from L2,
>> vmx_preemption_timer_value has to be updated according to the saved
>> hardware state. The corresponding code is missing in your patch so far.
>
> I think something else maybe be missing here: assuming L0 handles exits
> for L2 without involving L1 (e.g. external interrupts or ept violations),
> then, we may spend some cycles in L0 handling these exits. Note L1 is not
> aware of these exits and from L1 perspective L2 was running on the CPU.
> That means that we may need to reduce these cycles spent at
> L0 from the preemtion timer or emulate a preemption timer exit to
> force a transition to L1 instead of resuming L2.
My solution is setting "save preemption value" feature of L2 if L2
sets "vmx preemption timer" feature, thus external interrupts (or
others) will save the exact value in L2's vmcs, and the resume of L2
will load the value in L2's vmcs. Thus cycles of handling these vmexit
in L0 will not affect L2's preemption value.
Arthur
>
>
>
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