[libvirt-users] RLIMIT_MEMLOCK in container environment

[Ihar Hrachyshka]

On Thu, Aug 22, 2019 at 12:01 PM Laine Stump <laine at redhat.com> wrote:
>
> On 8/22/19 10:56 AM, Ihar Hrachyshka wrote:
> > On Thu, Aug 22, 2019 at 2:24 AM Daniel P. Berrangé <berrange at redhat.com> wrote:
> >>
> >> On Wed, Aug 21, 2019 at 01:37:21PM -0700, Ihar Hrachyshka wrote:
> >>> Hi all,
> >>>
> >>> KubeVirt uses libvirtd to manage qemu VMs represented as Kubernetes
> >>> API resources. In this case, libvirtd is running inside an
> >>> unprivileged pod, with some host mounts / capabilities added to the
> >>> pod, needed by libvirtd and other services.
> >>>
> >>> One of the capabilities libvirtd requires for successful startup
> >>> inside a pod is SYS_RESOURCE. This capability is used to adjust
> >>> RLIMIT_MEMLOCK ulimit value depending on devices attached to the
> >>> managed guest, both on startup and during hotplug. AFAIU the need to
> >>> lock the memory is to avoid pages being pushed out from RAM into swap.
>
>
> I recall successfully testing GPU assignment from an unprivileged
> libvirtd several years ago by setting a high enough ulimit for the uid
> used to run libvirtd in advance (. I think we check if the current
> setting is high enough, and don't try to set it unless we think we need to.
>

The PR I linked to in the original email does just that: it starts
libvirtd; then, if domain is going to use VFIO, sets ulimit of
libvirtd process to VM memory size + 1Gb (mimicking libvirt code) +
256Mb (to stay conservative) using prlimit() syscall; then defines the
domain.

> If I understand you correctly, you're saying that in your case it's okay
> for the memlock limit to be lower than we try to set it to, because swap
> is disabled anyway, is that correct?
>

I'm honestly not exactly sure about the reason why we need to set the
limit, but I assume it's because of swap. I can be totally confused on
that part though.

> >>
> >> Libvirt shouldn't set RLIMIT_MEMLOCK by default, unless there's
> >> something in the XML that requires it - one of
> >
> > You are right, sorry. We add SYS_RESOURCE only for particular domains.
> >
> >>
> >>   - hard limit memory value is present
> >>   - host PCI device passthrough is requested
> >
> > We are using passthrough
>
> (If you want to make Alex happy, use the term "VFIO device assignment"
> rather than passthrough :-).)
>

Not sure who Alex is but I'll try to make everyone happy! :)

> > to pass SR-IOV NIC VFs into guests. We also
> > plan to do the same for GPUs in the near future.
>
>  >>> I believe we would benefit from one of the following features on
>  >>> libvirt side (or both):
>  >>>
>  >>> a) expose the memory lock value calculated by libvirtd through
>  >>> libvirt ABI so that we can use it when calling prlimit() on libvirtd
>  >>> process;
>  >>> b) allow to disable setrlimit() calls via libvirtd config file knob
>  >>> or domain definition.
>
> (b) sounds much more reasonable, as long as qemu doesn't complain (I
> don't know whether or not it checks)
>
> Slightly related to this - I'm currently working on patches to avoid
> making any ioctl calls that would fail in an unprivileged libvirtd when
> using tap/macvtap devices. ATM, I'm doing this by adding an attribute
> "unmanaged='yes'" to the interface <target> element. The idea is that if
> someone sets unmanaged='yes', they're stating that the caller (i.e.
> kubevirt) is responsible for all device setup, and that libvirt should
> just use it without further setup. A similar approach could be applied
> to hostdev devices - if unmanaged is set, we assume that the caller has
> done everything to make the associated device usable.
>
> (Of course this all makes me realize the inanity of adding a <target
> dev='blah' unmanaged='yes'/> for interfaces when hostdevs already have
> <hostdev managed='yes'> and <interface type='hostdev' managed='yes'>. So
> to prevent setting the locklimit for hostdev, would we make a new
> setting like <hostdev managed='no-never-not-even-a-tiny-bit'>? Sigh. I
> *hate* trying to make config consistent :-/)
>
> (alternately, we could just automatically fail the attempt to set the
> lock limit in a graceful manner and allow the guest to continue)
>

If that's something maintainers feel good about, I am all for it since
it simplifies the implementation.

> BTW, I'm guessing that you use <hostdev> to assign the SRIOV VFs rather
> than <interface type='hostdev'>, correct? The latter would require that
> you have enough capabilities to set MAC addresses on the VFs (that's the
> entire point of using <interface type='hostdev'> instead of plain <hostdev>)

Yes, we use <hostdev> exactly because interface sets MAC address: in
kubevirt scenario, the container that is running libvirtd has its own
network namespace and doesn't have access to PF to set the VF MAC
address on. Instead, we rely on CNI plugin that is running in the root
namespace context to configure the VF interface as needed. (I've
contributed custom MAC support to SR-IOV CNI plugin very recently.)

Ihar