[RFC PATCH 1/5] ARM/ARM64: KVM: Update user space API header for PSCI emulation

[Marc Zyngier]

On 17/10/13 12:49, Alexander Graf wrote:
> 
> On 17.10.2013, at 13:30, Anup Patel <anup at brainfault.org> wrote:
> 
>> On Thu, Oct 17, 2013 at 4:51 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>> On 17/10/13 12:10, Anup Patel wrote:
>>>> On Thu, Oct 17, 2013 at 2:17 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>>>> On 17/10/13 07:45, Anup Patel wrote:
>>>>>> On Thu, Oct 17, 2013 at 3:41 AM, Christoffer Dall
>>>>>> <christoffer.dall at linaro.org> wrote:
>>>>>>> On Wed, Oct 16, 2013 at 10:32:30PM +0530, Anup Patel wrote:
>>>>>>>> Update user space API interface headers for providing information to
>>>>>>>> user space needed to emulate PSCI function calls in user space (i.e.
>>>>>>>> QEMU or KVMTOOL).
>>>>>>>>
>>>>>>>> Signed-off-by: Anup Patel <anup.patel at linaro.org>
>>>>>>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar at linaro.org>
>>>>>>>> ---
>>>>>>>> include/uapi/linux/kvm.h |    7 +++++++
>>>>>>>> 1 file changed, 7 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>>>>>>> index e32e776..dae2664 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_PSCI             24
>>>>>>>>
>>>>>>>> /* For KVM_EXIT_INTERNAL_ERROR */
>>>>>>>> /* Emulate instruction failed. */
>>>>>>>> @@ -301,6 +302,12 @@ struct kvm_run {
>>>>>>>>              struct {
>>>>>>>>                      __u32 epr;
>>>>>>>>              } epr;
>>>>>>>> +             /* KVM_EXIT_PSCI */
>>>>>>>> +             struct {
>>>>>>>> +                     __u32 fn;
>>>>>>>> +                     __u64 args[7];
>>>>>>>> +                     __u64 ret[4];
>>>>>>>> +             } psci;
>>>>>>>>              /* Fix the size of the union. */
>>>>>>>>              char padding[256];
>>>>>>>>      };
>>>>>>>> --
>>>>>>>> 1.7.9.5
>>>>>>>>
>>>>>>> I am also wondering if this is not solving a very specific need without
>>>>>>> thinking a little more carefully about this problem.
>>>>>>
>>>>>> No, its not solving a specific problem.
>>>>>>
>>>>>> In fact, its more general because we pass complete info required to
>>>>>> emulate a PSCI call in user space.
>>>>>> (Please refer PSCI calling convention)
>>>>>>
>>>>>>>
>>>>>>> We have previously discussed the need for some secure side emulation
>>>>>>> in QEMU, and I think perhaps we need something more generic which allows
>>>>>>> user space to handle SMC calls and/or allows user space to "inject" some
>>>>>>> secure world runtime that the kernel can run in a partially or fully
>>>>>>> isolated container to handle SMC calls.
>>>>>>>
>>>>>>> Peter raised this issue previously and pointed to a proposal he had as
>>>>>>> well.
>>>>>>
>>>>>> If required we can have an additional field in kvm_run->psci which tells
>>>>>> whether the PSCI call is an SMC call or HVC call.
>>>>>>
>>>>>>>
>>>>>>> Is there a technical reason why we need something specifically directed
>>>>>>> to PSCI?
>>>>>>
>>>>>> Its quite natural to add this to PSCI emulation in KVM ARM/ARM64 instead
>>>>>> of adding a separate VirtIO device for System reboot and System poweroff.
>>>>>>
>>>>>> Also in the process of implementing SYSTEM_OFF and SYSTEM_RESET
>>>>>> emulation in user space we would also have an infrastructure for adding
>>>>>> emulation of new PSCI calls in user space.
>>>>>
>>>>> And I strongly oppose to that. It creates consistency issues (what if
>>>>> userspace implements one version of PSCI, and the kernel another?), and
>>>>> also some really horrible situations: Imagine you implement the SUSPEND
>>>>> operation in userspace, and want to wake the vcpu up with an interrupt.
>>>>> You'd end-up having to keep track of the state in the kernel, having to
>>>>> forward the interrupt event to userspace...
>>>>
>>>> It is not about emulating all PSCI functions in user space. Its about forwarding
>>>> system-level PSCI functions or PSCI functions which cannot be emulated in
>>>> kernel to user space.
>>>
>>> The CPU parts of PSCI can perfectly be implemented in the kernel.
>>
>> Agreed. This patches does the same.
>>
>>>
>>> Then you can return something to userspace indicating what just
>>> happened. And it doesn't have to be PSCI specific.
>>
>> Are you suggesting that everytime we want to emulate some new
>> PSCI call with help from user space (e.g. SYSTEM_OFF and
>> SYSTEM_RESET), we add new exit reasons and just keep on
>> increasing KVM exit reasons ?
>>
>> Why can't the exit reason and exit info in struct kvm_run be
>> PSCI specific ?
>>
>> On the contrary, it will be good to have exit reason and exit info
>> PSCI specific because we have PSCI specification which tells
>> how it is to be emulated ?
> 
> I completely agree with Marc that split-brain ownership of any address space (and PSCI is basically one) is a very bad idea.
> 
> However, so far the only solution I've seen mentioned is that the kernel owns PSCI (read: decodes it) and then drives user space with explicit commands.
> 
> Couldn't we reverse this logic? User space owns PSCI. By default all PSCI calls go to user space. If a PSCI call makes more sense to be executed by kvm, it can explicitly route it to be handled by kvm instead.
> 
> That way the owner is still at a single spot and we can fast path the few cases that may be performance critical or a lot easier to handle in kvm.
> 
> The good part about this is that we get consistency in QEMU with the TCG PSCI handlers along the way.

The only nag here is that you can't do that for every function: SUSPEND
is one, for example. Once your vcpu is suspended, you need to to wake it
up with an interrupt, which are not routed to userspace (TFFT!).

So it becomes yet another can of worms, and I rather keep it simple.

	M.
-- 
Jazz is not dead. It just smells funny...