Re: [PATCH 1/5] KVM: arm64: Document PV-lock interface

[Steven Price <steven.price@arm.com>]

On 19/12/2019 11:45, yezengruan wrote:
> Hi Steve,
> 
> On 2019/12/17 22:21, Steven Price wrote:
>> On Tue, Dec 17, 2019 at 01:55:45PM +0000, yezengruan@huawei.com wrote:
>>> From: Zengruan Ye <yezengruan@huawei.com>
>>>
>>> Introduce a paravirtualization interface for KVM/arm64 to obtain the vcpu
>>> is currently running or not.
>>>
>>> A hypercall interface is provided for the guest to interrogate the
>>> hypervisor's support for this interface and the location of the shared
>>> memory structures.
>>>
>>> Signed-off-by: Zengruan Ye <yezengruan@huawei.com>
>>> ---
>>>  Documentation/virt/kvm/arm/pvlock.rst | 31 +++++++++++++++++++++++++++
>>>  1 file changed, 31 insertions(+)
>>>  create mode 100644 Documentation/virt/kvm/arm/pvlock.rst
>>>
>>> diff --git a/Documentation/virt/kvm/arm/pvlock.rst b/Documentation/virt/kvm/arm/pvlock.rst
>>> new file mode 100644
>>> index 000000000000..eec0c36edf17
>>> --- /dev/null
>>> +++ b/Documentation/virt/kvm/arm/pvlock.rst
>>> @@ -0,0 +1,31 @@
>>> +.. SPDX-License-Identifier: GPL-2.0
>>> +
>>> +Paravirtualized lock support for arm64
>>> +======================================
>>> +
>>> +KVM/arm64 provids some hypervisor service calls to support a paravirtualized
>>> +guest obtaining the vcpu is currently running or not.
>>> +
>>> +Two new SMCCC compatible hypercalls are defined:
>>> +
>>> +* PV_LOCK_FEATURES:   0xC5000040
>>> +* PV_LOCK_PREEMPTED:  0xC5000041
>>
>> These values are in the "Standard Hypervisor Service Calls" section of
>> SMCCC - so is there a document that describes this features such that
>> other OSes or hypervisors can implement it? I'm also not entirely sure
>> of the process of ensuring that the IDs picked are non-conflicting.
>>
>> Otherwise if this is a KVM specific interface this should probably
>> belong within the "Vendor Specific Hypervisor Service Calls" section
>> along with some probing that the hypervisor is actually KVM. Although I
>> don't see anything KVM specific.
> 
> Thanks for pointing it out to me! Actually, I also don't see any documents
> or KVM specific that describes this features. The values in the "Vendor
> Specific Hypervisor Service Calls" section may be more appropriate, such as
> the following
> 
> * PV_LOCK_FEATURES:   0xC6000020
> * PV_LOCK_PREEMPTED:  0xC6000021
> 
> Please let me know if you have any suggestions.

I don't have strong feelings on whether this should be KVM-specific or
generic. I'm not familiar with whether there are competing solutions to
this problem - it's obviously ideal if all hypervisors can make use of
the same interface if possible, but maybe that ship has sailed already?

However if this going to be KVM-specific then you'll need to add the
probing logic for checking whether the hypervisor is KVM or not. Will
has a couple of patches on a branch which do this [1] and [2]. Then you
can use kvm_arm_hyp_services_available() as the first step to probe
whether the hypervisor is KVM.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/commit/?h=kvm/hvc&id=464f5a1741e5959c3e4d2be1966ae0093b4dce06

[2]
https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/commit/?h=kvm/hvc&id=6597490e005d0eeca8ed8c1c1d7b4318ee014681

Steve

>>
>>> +
>>> +The existence of the PV_LOCK hypercall should be probed using the SMCCC 1.1
>>> +ARCH_FEATURES mechanism before calling it.
>>> +
>>> +PV_LOCK_FEATURES
>>> +    ============= ========    ==========
>>> +    Function ID:  (uint32)    0xC5000040
>>> +    PV_call_id:   (uint32)    The function to query for support.
>>> +    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
>>> +                              PV-lock feature is supported by the hypervisor.
>>> +    ============= ========    ==========
>>> +
>>> +PV_LOCK_PREEMPTED
>>> +    ============= ========    ==========
>>> +    Function ID:  (uint32)    0xC5000041
>>> +    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the IPA of
>>> +                              this vcpu's pv data structure is configured by
>>> +                              the hypervisor.
>>> +    ============= ========    ==========
>>
>> >From the code it looks like there's another argument for this SMC - the
>> physical address (or IPA) of a struct pvlock_vcpu_state. This structure
>> also needs to be described as it is part of the ABI.
> 
> Will update.
> 
>>
>> Steve
>>
>> .
>>
> 
> Thanks,
> 
> Zengruan
> 
> 
> 
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