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

[Andrew Jones <drjones@redhat.com>]

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?

> 
> >>
> >>>> +  Stolen time |      8      |      8      | Stolen time in unsigned
> >>>> +              |             |             | nanoseconds indicating how
> >>>> +              |             |             | much time this VCPU thread
> >>>> +              |             |             | was involuntarily not
> >>>> +              |             |             | running on a physical CPU.
> >>>> +
> >>>> +The structure will be updated by the hypervisor prior to scheduling a VCPU. It
> >>>> +will be present within a reserved region of the normal memory given to the
> >>>> +guest. The guest should not attempt to write into this memory. There is a
> >>>> +structure per VCPU of the guest.
> >>>
> >>> Should we provide a recommendation as to how that reserved memory is
> >>> provided? One memslot divided into NR_VCPUS subregions? Should the
> >>> reserved region be described to the guest kernel with DT/ACPI? Or
> >>> should userspace ensure the region is not within any DT/ACPI described
> >>> regions?
> >>
> >> I'm open to providing a recommendation, but I'm not entirely sure I know
> >> enough here to provide one.
> >>
> >> There is an obvious efficiency argument for minimizing memslots with the
> >> current code. But if someone has a reason for using multiple memslots
> >> then that's probably a good argument for implementing a memslot-caching
> >> kvm_put_user() rather than to be dis-recommended.
> > 
> > Actually even if a single memslot is used for all the PV structures for
> > all VCPUs, but it's separate from the slot(s) used for main memory, then
> > we'll likely see performance issues with memslot searches (even though
> > it's a binary search). This is because memslots already have caching. The
> > last used slot is stored in the memslots' lru_slot member (the "lru" name
> > is confusing, but it means "last used" somehow). This means we could get
> > thrashing on that slot cache if we're searching for the PV structure
> > memslot on each vcpu load after searching for the main memory slot on each
> > page fault.
> 
> True - a dedicated memslot for stolen time wouldn't be great if a VM is
> needing to fault pages (which would obviously be in a different
> memslot). I don't have a good idea of the overhead of missing in the
> lru_slot cache. The main reason I stopped using a dedicated cache was
> because I discovered that my initial implementation using
> kvm_write_guest_offset_cached() (which wasn't single-copy atomic safe)
> was actually failing to use the cache because the buffer crossed a page
> boundary (see __kvm_gfn_to_hva_cache_init()). So switching away from the
> "_cached" variant was actually avoiding the extra walks of the memslots.
> 
> I can look at reintroducing the caching for kvm_put_guest().
> 
> >>
> >> My assumption (and testing) has been with a single memslot divided into
> >> NR_VCPUS (or more accurately the number of VCPUs in the VM) subregions.
> >>
> >> For testing DT I've tested both methods: an explicit reserved region or
> >> just ensuring it's not in any DT described region. Both seem reasonable,
> >> but it might be easier to integrate into existing migration mechanisms
> >> if it's simply a reserved region (then the memory block of the guest is
> >> just as it always was).
> >>
> >> For ACPI the situation should be similar, but my testing has been with DT.
> > 
> > I also can't think of any reason why we'd have to describe it in DT/ACPI,
> > but I get this feeling that if we don't, then we'll hit some issue that
> > will make us wish we had...
> 
> Without knowing why we need it it's hard to justify what should go in
> the bindings. But the idea of having the hypercalls is that the
> description is returned via hypercalls rather than explicitly in
> DT/ACPI. In theory we wouldn't need the hypercalls if it was fully
> described in DT/ACPI.
>

Fair enough

Thanks,
drew