Re: [PATCH v4 01/10] KVM: arm64: Document PV-time interface

[Steven Price]

On 04/09/2019 15:22, Andrew Jones wrote:
> On Wed, Sep 04, 2019 at 02:55:15PM +0100, Steven Price wrote:
>> On 02/09/2019 13:52, Andrew Jones wrote:
>>> On Fri, Aug 30, 2019 at 04:25:08PM +0100, Steven Price wrote:
>>>> On 30/08/2019 15:47, Andrew Jones wrote:
>>>>> On Fri, Aug 30, 2019 at 09:42:46AM +0100, Steven Price wrote:
>> [...]
>>>>>> +    Return value: (int32)   : NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
>>>>>> +                              PV-time feature is supported by the hypervisor.
>>>>>> +
>>>>>> +PV_TIME_ST
>>>>>> +    Function ID:  (uint32)  : 0xC5000022
>>>>>> +    Return value: (int64)   : IPA of the stolen time data structure for this
>>>>>> +                              VCPU. On failure:
>>>>>> +                              NOT_SUPPORTED (-1)
>>>>>> +
>>>>>> +The IPA returned by PV_TIME_ST should be mapped by the guest as normal memory
>>>>>> +with inner and outer write back caching attributes, in the inner shareable
>>>>>> +domain. A total of 16 bytes from the IPA returned are guaranteed to be
>>>>>> +meaningfully filled by the hypervisor (see structure below).
>>>>>> +
>>>>>> +PV_TIME_ST returns the structure for the calling VCPU.
>>>>>> +
>>>>>> +Stolen Time
>>>>>> +-----------
>>>>>> +
>>>>>> +The structure pointed to by the PV_TIME_ST hypercall is as follows:
>>>>>> +
>>>>>> +  Field       | Byte Length | Byte Offset | Description
>>>>>> +  ----------- | ----------- | ----------- | --------------------------
>>>>>> +  Revision    |      4      |      0      | Must be 0 for version 0.1
>>>>>> +  Attributes  |      4      |      4      | Must be 0
>>>>>
>>>>> The above fields don't appear to be exposed to userspace in anyway. How
>>>>> will we handle migration from one KVM with one version of the structure
>>>>> to another?
>>>>
>>>> Interesting question. User space does have access to them now it is
>>>> providing the memory, but it's not exactly an easy method. In particular
>>>> user space has no (simple) way of probing the kernel's supported version.
>>>>
>>>> I guess one solution would be to add an extra attribute on the VCPU
>>>> which would provide the revision information. The current kernel would
>>>> then reject any revision other than 0, but this could then be extended
>>>> to support other revision numbers in the future.
>>>>
>>>> Although there's some logic in saying we could add the extra attribute
>>>> when(/if) there is a new version. Future kernels would then be expected
>>>> to use the current version unless user space explicitly set the new
>>>> attribute.
>>>>
>>>> Do you feel this is something that needs to be addressed now, or can it
>>>> be deferred until another version is proposed?
>>>
>>> Assuming we'll want userspace to have the option of choosing version=0,
>>> and that we're fine with version=0 being the implicit choice, when nothing
>>> is selected, then I guess it can be left as is for now. If, OTOH, we just
>>> want migration to fail when attempting to migrate to another host with
>>> an incompatible stolen-time structure (i.e. version=0 is not selectable
>>> on hosts that implement later versions), then we should expose the version
>>> in some way now. Perhaps a VCPU's "PV config" should be described in a
>>> set of pseudo registers?
>>
>> I wouldn't have thought making migration fail if/when the host upgrades
>> to a new version would be particularly helpful - we'd want to provide
>> backwards compatibility. In particular for the suspend/resume case (I
>> want to be able to save my VM to disk, upgrade the host kernel and then
>> resume the VM).
>>
>> The only potential issue I see is the implicit "version=0 if not
>> specified". That seems solvable by rejecting setting the stolen time
>> base address if no version has been specified and the host kernel
>> doesn't support version=0.
> 
> I think that's the same failure I was trying avoid by failing the
> migration instead. Maybe it's equivalent to fail at this vcpu-ioctl
> time though?

Yes this is effectively the same failure. But since we require the
vcpu-ioctl to enable stolen time this gives an appropriate place to
fail. Indeed this is the failure if migrating from a host with these
patches to one running an existing kernel with no stolen time support.

Steve