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

[Anup Patel]

On Tue, Dec 10, 2013 at 9:19 PM, Alexander Graf <agraf at suse.de> wrote:
>
> 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.

Thanks for the info on the x86 part.

I think all this info should have been part of KVM api documentation
for KVM_EXIT_SHUTDOWN.

>
>>
>>>
>>> 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.

Sure, it makes sense to avoid use of KVM_EXIT_SHUTDOWN
in arm/arm64.

How about adding exit reasons KVM_EXIT_SYSTEM_RESET
and KVM_EXIT_SYSTEM_SHUTDOWN ?

These exit reasons will be used for arm/arm64 but can also be
used by other architectures if they want.

--
Anup

>
>
> Alex
>