kvm-owner@xxxxxxxxxxxxxxx wrote on 25/08/2013 10:43:12 AM:
> From: Jan Kiszka <jan.kiszka@xxxxxx>
> To: Abel Gordon/Haifa/IBM@IBMIL,
> Cc: gleb@xxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, kvm-owner@xxxxxxxxxxxxxxx,
> pbonzini@xxxxxxxxxx, "李春奇 <Arthur Chunqi Li>" <yzt356@xxxxxxxxx>
> Date: 25/08/2013 10:43 AM
> Subject: Re: [PATCH] KVM: nVMX: Fully support of nested VMX preemption
timer
> Sent by: kvm-owner@xxxxxxxxxxxxxxx
>
> On 2013-08-25 09:37, Abel Gordon 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.
>
> That's precisely what the logic I described should achieve: reload the
> value we saved on L2 exit on reentry.
But don't you think we should also reduce the cycles spent at L0 from the
preemption timer ? I mean, if we spent X cycles at L0 handling a L2 exit
which was not forwarded to L1, then, before we resume L2,
the preemption timer should be: (previous_value_on_exit - X).
If (previous_value_on_exit - X) < 0, then we should force ("emulate") a
preemption timer exit between L2 and L1.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
