libvirt users - Aug 2019 - Re: RLIMIT_MEMLOCK in container environment

On Thu, Aug 22, 2019 at 12:01 PM Laine Stump <laine@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@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

(Adding Alex Williamson to Cc so he can correct any mistakes)

On 8/22/19 4:39 PM, Ihar Hrachyshka wrote:
> On Thu, Aug 22, 2019 at 12:01 PM Laine Stump <laine@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@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.
So you're making an educated guess, which is essentially what libvirt is 
doing (based on advice from other people with better information than 
us, but still a guess).
> 
>> 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.

What I understand from an IRC conversation with Alex just now is that 
increasing RLIMIT_MEMLOCK isn't done just to prevent any of the pages 
being swapped out. It's done because "all GPAs (Guest Physical 
Addresses) that could potentially be DMA targets need to have fixed 
mappings through the iommu, therefore all need to be allocated and 
mappings fixed [...] setting rlimit allows us to perform all the 
necessary pins within the user's locked memory limit".

So even if swap is disabled, it still needs to be done (either by 
libvirt, or by someone else who has the necessary privileges and control 
over the libvirtd process).

>>>>
>>>> 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! :)
The Alex I'm referring to is the Alex I just Cc'ed. He is the VFIO 
maintainer.

>>> 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.
Well, after talking to Alex, I think that since a) libvirt only attempts 
to increase the limit after determining that it isn't already high 
enough, and b) if it isn't high enough and we can't increase it, then 
qemu is going to fail anyway, that c) we can't just fail gracefully and 
continue.

So *somebody* needs to increase the limit, and if you want libvirt to be 
unprivileged, that means it needs to be you doing the increase. And 
since the amount that libvirt increases it is just some number based on 
oral folklore (and not on a specific value we learn by querying 
somewhere), I don't think it's worthwhile figuring out some way for 
libvirt to report it via an official API - that would end up just being 
this:

"Hey, you know that number that you guys are just making a guess about 
based on some advice someone gave you once? Yeah, send me *that* number 
so I can claim to be basing my actions on real science instead of 
slightly educated voodoo! K THX BYE!" :-)

> 
>> 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
>

On Fri, 23 Aug 2019, 0:27 Laine Stump, <laine@redhat.com> wrote:
> (Adding Alex Williamson to Cc so he can correct any mistakes)
>
> On 8/22/19 4:39 PM, Ihar Hrachyshka wrote:
> > On Thu, Aug 22, 2019 at 12:01 PM Laine Stump <laine@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@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.
>
> So you're making an educated guess, which is essentially what libvirt
is
> doing (based on advice from other people with better information than
> us, but still a guess).
>
> >
> >> 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.
>
>
> What I understand from an IRC conversation with Alex just now is that
> increasing RLIMIT_MEMLOCK isn't done just to prevent any of the pages
> being swapped out. It's done because "all GPAs (Guest Physical
> Addresses) that could potentially be DMA targets need to have fixed
> mappings through the iommu, therefore all need to be allocated and
> mappings fixed [...] setting rlimit allows us to perform all the
> necessary pins within the user's locked memory limit".
>
> So even if swap is disabled, it still needs to be done (either by
> libvirt, or by someone else who has the necessary privileges and control
> over the libvirtd process).
>
>
> >>>>
> >>>> 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! :)
>
> The Alex I'm referring to is the Alex I just Cc'ed. He is the VFIO
> maintainer.
>
>
> >>> 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.

This is music to my ears, great to hear.

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 :-/)
>
Sounds tough indeed. I'd try to avoid negatively-named knobs. managed=no is
simpler to perceive than unmanaged=yes. It may be just me, but I'd even
assume managed=no if the target dev name is specified. If libvirt manages
the tap device, it should create a fresh one, too. But all of this is a big
digression.
>>
> >> (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.
>
> Well, after talking to Alex, I think that since a) libvirt only attempts
> to increase the limit after determining that it isn't already high
> enough, and b) if it isn't high enough and we can't increase it,
then
> qemu is going to fail anyway, that c) we can't just fail gracefully and
> continue.
>
> So *somebody* needs to increase the limit, and if you want libvirt to be
> unprivileged, that means it needs to be you doing the increase. And
> since the amount that libvirt increases it is just some number based on
> oral folklore (and not on a specific value we learn by querying
> somewhere), I don't think it's worthwhile figuring out some way for
> libvirt to report it via an official API - that would end up just being
> this:
>
> "Hey, you know that number that you guys are just making a guess about
> based on some advice someone gave you once? Yeah, send me *that* number
> so I can claim to be basing my actions on real science instead of
> slightly educated voodoo! K THX BYE!" :-)
>
Well, it's more like: "you know that voodoo you do to guess the number?
If
you ever educate yourself about it, e.g by querying qemu, send me *that*
number. I'd rather not think about it ever again, BYE."

> >
> >> 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
> >
>
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