[PATCH 1/2] KVM: Add KVM_EXIT_RESET to user space API header

[Alexander Graf]

On 10.12.2013, at 05:23, Anup Patel <anup at brainfault.org> wrote:

> On Tue, Dec 10, 2013 at 7:43 AM, Alexander Graf <agraf at suse.de> wrote:
>> 
>> On 25.11.2013, at 16:49, Anup Patel <anup.patel at linaro.org> wrote:
>> 
>>> Currently, we don't have an exit reason for VM reset emulation
>>> in user space hence this patch adds exit reason KVM_EXIT_RESET
>>> for this purpose.
>>> 
>>> This newly added KVM_EXIT_RESET will be used by KVM arm/arm64
>>> in-kernel PSCI support to reset VMs.
>>> 
>>> Signed-off-by: Anup Patel <anup.patel at linaro.org>
>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar at linaro.org>
>>> ---
>>> include/uapi/linux/kvm.h |    1 +
>>> 1 file changed, 1 insertion(+)
>>> 
>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>> index 902f124..64a04cc 100644
>>> --- a/include/uapi/linux/kvm.h
>>> +++ b/include/uapi/linux/kvm.h
>>> @@ -171,6 +171,7 @@ struct kvm_pit_config {
>>> #define KVM_EXIT_WATCHDOG         21
>>> #define KVM_EXIT_S390_TSCH        22
>>> #define KVM_EXIT_EPR              23
>>> +#define KVM_EXIT_RESET            24
>> 
>> I have to admit that I'm not particularly happy with the exit name. It's not obvious from the name under which circumstances it gets triggered. Does it get triggered when a core level reset happens? Does it get triggered when a system level reset happened? When the guest requests one?
> 
> The KVM_EXIT_RESET gets triggered when system level reset is
> initiated by VCPU. For arm/arm64, this is through SYSTEM_RESET
> PSCI call. In KVM x86 SVM/VMX, we have KVM_EXIT_SHUTDOWN
> being used for system shutdown which we have re-used for arm/arm64.

Yeah, that name already did mislead you once :).

KVM_EXIT_SHUTDOWN happens on

  * triple fault
  * CPU internal severe problems

the latter is defined as:

In contrast, an error that cannot be contained and is of such severity that it has compromised the continued operation of a processor core requires immediate action to terminate system processing and may result in a hardware-enforced shutdown. In the shutdown state, the execution of instructions by that processor core is halted. See Section 8.2.9 “#DF—Double-Fault Exception (Vector 8)” on page 220 for a description of the shutdown processor state.

Triple faults are used commonly in 286 code to switch from PG to real mode. So they _have_ to be emulated as core reset. Otherwise you break old guests.

However, the scope of this exit is definitely vcpu wide. What you are looking for is a system wide notification. Commonly this happens through MMIO, but I can see why you wouldn't want that with PSCI interpreted in the kernel. That's why I asked you to create a completely new one to not add up the the confusion.

> 
>> 
>> I know what it does, but I find the name too generic for what it is. What you're really doing is introduce a new communication channel in parallel to MMIO / PIO / HCALL which is only used for system level reset / shutdown today.
>> 
>> Can we treat it as such? Could you please make this a common exit number that's called something like
>> 
>>  KVM_EXIT_SYSTEM_EVENT
>> 
>> with a parameter that can either be TRIGGER_SHUTDOWN or TRIGGER_RESET.
>> 
>> That way it's obvious what's going on and people don't get confused.
> 
> I don't foresee any system level operations other than SHUTDOWN
> and RESET to be handled from KVM in-kernel code but I might be
> wrong.

The good but about the EXIT_SYSTEM_EVENT is that it's immediately obvious that we're not talking about a vcpu local event. But I'm open to better names.

> May be we can rename KVM_EXIT_SHUTDOWN and KVM_EXIT_RESET
> to KVM_EXIT_SYSTEM_SHUTDOWN and KVM_EXIT_SYSTEM_RESET ??

You definitely can not rename KVM_EXIT_SHUTDOWN. It's part of the KVM API. In fact, I think it's a bad idea to even reuse the name as it clearly works on vcpu level.


Alex