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
