Re: [PATCH v5 1/4] domctl: introduce a new domain create flag, XEN_DOMCTL_CDF_evtchn_fifo, ...

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.12.2020 16:45, Paul Durrant wrote:
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 03 December 2020 15:23
>>
>> On 03.12.2020 13:41, Paul Durrant wrote:
>>> From: Paul Durrant <pdurrant@xxxxxxxxxx>
>>>
>>> ...to control the visibility of the FIFO event channel operations
>>> (EVTCHNOP_init_control, EVTCHNOP_expand_array, and EVTCHNOP_set_priority) to
>>> the guest.
>>>
>>> These operations were added to the public header in commit d2d50c2f308f
>>> ("evtchn: add FIFO-based event channel ABI") and the first implementation
>>> appeared in the two subsequent commits: edc8872aeb4a ("evtchn: implement
>>> EVTCHNOP_set_priority and add the set_priority hook") and 88910061ec61
>>> ("evtchn: add FIFO-based event channel hypercalls and port ops"). Prior to
>>> that, a guest issuing those operations would receive a return value of
>>> -ENOSYS (not implemented) from Xen. Guests aware of the FIFO operations but
>>> running on an older (pre-4.4) Xen would fall back to using the 2-level event
>>> channel interface upon seeing this return value.
>>>
>>> Unfortunately the uncontrolable appearance of these new operations in Xen 
>>> 4.4
>>> onwards has implications for hibernation of some Linux guests. During resume
>>> from hibernation, there are two kernels involved: the "boot" kernel and the
>>> "resume" kernel. The guest boot kernel may default to use FIFO operations 
>>> and
>>> instruct Xen via EVTCHNOP_init_control to switch from 2-level to FIFO. On 
>>> the
>>> other hand, the resume kernel keeps assuming 2-level, because it was 
>>> hibernated
>>> on a version of Xen that did not support the FIFO operations.
>>>
>>> To maintain compatibility it is necessary to make Xen behave as it did
>>> before the new operations were added and hence the code in this patch 
>>> ensures
>>> that, if XEN_DOMCTL_CDF_evtchn_fifo is not set, the FIFO event channel
>>> operations will again result in -ENOSYS being returned to the guest.
>>
>> I have to admit I'm now even more concerned of the control for such
>> going into Xen, the more with the now 2nd use in the subsequent patch.
>> The implication of this would seem to be that whenever we add new
>> hypercalls or sub-ops, a domain creation control would also need
>> adding determining whether that new sub-op is actually okay to use by
>> a guest. Or else I'd be keen to up front see criteria at least roughly
>> outlined by which it could be established whether such an override
>> control is needed.
>>
> 
> Ultimately I think any new hypercall (or related set of hypercalls) added to 
> the ABI needs to be opt-in on a per-domain basis, so that we know that from 
> when a domain is first created it will not see a change in its environment 
> unless the VM administrator wants that to happen.

A new hypercall appearing is a change to the guest's environment, yes,
but a backwards compatible one. I don't see how this would harm a guest.
This and ...

>> I'm also not convinced such controls really want to be opt-in rather
>> than opt-out.
> 
> They really need to be opt-in I think. From a cloud provider PoV it is 
> important that nothing in a customer's environment changes unless we want it 
> to. Otherwise we have no way to deploy an updated hypervisor version without 
> risking crashing their VMs.

... this sound to me more like workarounds for buggy guests than
functionality the hypervisor _needs_ to have. (I can appreciate
the specific case here for the specific scenario you provide as
an exception.)

>>> --- a/xen/arch/arm/domain.c
>>> +++ b/xen/arch/arm/domain.c
>>> @@ -622,7 +622,8 @@ int arch_sanitise_domain_config(struct 
>>> xen_domctl_createdomain *config)
>>>      unsigned int max_vcpus;
>>>
>>>      /* HVM and HAP must be set. IOMMU may or may not be */
>>> -    if ( (config->flags & ~XEN_DOMCTL_CDF_iommu) !=
>>> +    if ( (config->flags &
>>> +          ~(XEN_DOMCTL_CDF_iommu | XEN_DOMCTL_CDF_evtchn_fifo) !=
>>>           (XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap) )
>>>      {
>>>          dprintk(XENLOG_INFO, "Unsupported configuration %#x\n",
>>> --- a/xen/arch/arm/domain_build.c
>>> +++ b/xen/arch/arm/domain_build.c
>>> @@ -2478,7 +2478,8 @@ void __init create_domUs(void)
>>>          struct domain *d;
>>>          struct xen_domctl_createdomain d_cfg = {
>>>              .arch.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE,
>>> -            .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap,
>>> +            .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
>>> +                     XEN_DOMCTL_CDF_evtchn_fifo,
>>>              .max_evtchn_port = -1,
>>>              .max_grant_frames = 64,
>>>              .max_maptrack_frames = 1024,
>>> --- a/xen/arch/arm/setup.c
>>> +++ b/xen/arch/arm/setup.c
>>> @@ -805,7 +805,8 @@ void __init start_xen(unsigned long boot_phys_offset,
>>>      struct bootmodule *xen_bootmodule;
>>>      struct domain *dom0;
>>>      struct xen_domctl_createdomain dom0_cfg = {
>>> -        .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap,
>>> +        .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
>>> +                 XEN_DOMCTL_CDF_evtchn_fifo,
>>>          .max_evtchn_port = -1,
>>>          .max_grant_frames = gnttab_dom0_frames(),
>>>          .max_maptrack_frames = -1,
>>> --- a/xen/arch/x86/setup.c
>>> +++ b/xen/arch/x86/setup.c
>>> @@ -738,7 +738,8 @@ static struct domain *__init create_dom0(const module_t 
>>> *image,
>>>                                           const char *loader)
>>>  {
>>>      struct xen_domctl_createdomain dom0_cfg = {
>>> -        .flags = IS_ENABLED(CONFIG_TBOOT) ? XEN_DOMCTL_CDF_s3_integrity : 
>>> 0,
>>> +        .flags = XEN_DOMCTL_CDF_evtchn_fifo |
>>> +                 (IS_ENABLED(CONFIG_TBOOT) ? XEN_DOMCTL_CDF_s3_integrity : 
>>> 0),
>>>          .max_evtchn_port = -1,
>>>          .max_grant_frames = -1,
>>>          .max_maptrack_frames = -1,
>>> --- a/xen/common/domain.c
>>> +++ b/xen/common/domain.c
>>> @@ -307,7 +307,7 @@ static int sanitise_domain_config(struct 
>>> xen_domctl_createdomain *config)
>>>           ~(XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
>>>             XEN_DOMCTL_CDF_s3_integrity | XEN_DOMCTL_CDF_oos_off |
>>>             XEN_DOMCTL_CDF_xs_domain | XEN_DOMCTL_CDF_iommu |
>>> -           XEN_DOMCTL_CDF_nested_virt) )
>>> +           XEN_DOMCTL_CDF_nested_virt | XEN_DOMCTL_CDF_evtchn_fifo) )
>>>      {
>>>          dprintk(XENLOG_INFO, "Unknown CDF flags %#x\n", config->flags);
>>>          return -EINVAL;
>>
>> All of the hunks above point out a scalability issue if we were to
>> follow this route for even just a fair part of new sub-ops, and I
>> suppose you've noticed this with the next patch presumably touching
>> all the same places again.
> 
> Indeed. This solution works for now but is probably not what we want in the 
> long run. Same goes for the current way we control viridian features via an 
> HVM param. It is good enough for now IMO since domctl is not a stable 
> interface. Any ideas about how we might implement a better interface in the 
> longer term?

While it has other downsides, Jürgen's proposal doesn't have any
similar scalability issue afaics. Another possible model would
seem to be to key new hypercalls to hypervisor CPUID leaf bits,
and derive their availability from a guest's CPUID policy. Of
course this won't work when needing to retrofit guarding like
you want to do here.

Jan