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

From: [email-masked] (Marc Zyngier)
Date: Thu, 17 Oct 2013 12:21:17 +0100

On 17/10/13 12:10, Anup Patel wrote:
> On Thu, Oct 17, 2013 at 2:17 PM, Marc Zyngier <[email-masked]> wrote:
>> On 17/10/13 07:45, Anup Patel wrote:
>>> On Thu, Oct 17, 2013 at 3:41 AM, Christoffer Dall
>>> <[email-masked]> 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 <[email-masked]>
>>>>> Signed-off-by: Pranavkumar Sawargaonkar <[email-masked]>
>>>>> ---
>>>>>  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.

Then you can return something to userspace indicating what just
happened. And it doesn't have to be PSCI specific.

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