On 8/24/19 3:08 AM, Dan Kenigsberg wrote:
On Fri, 23 Aug 2019, 0:27 Laine Stump, <la...@redhat.com
<mailto:la...@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 <la...@redhat.com
<mailto:la...@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
<berra...@redhat.com <mailto:berra...@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.
Yeah, I don't like double negatives either, but since the default needs
to preserve existing behavior, and it's easier for a default setting to
be "no" rather than "yes"...,
Still, I'm not married to this name, just using it so that I can get
*something* going.
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.
If we were starting from scratch, that's what I would prefer too. The
only problem is that we have to maintain existing behavior for current
users. The way that it works currently is that if you specify the tap
device name and it exists, then libvirt will still set the MAC address
and some IFF_* flags. If we suddenly stop doing that, then the existing
users' configs will be broken :-/
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."
I can see the motivation. But that assumes that qemu knows the right
answer, and had a way to query it. If they ever do that, then maybe we
could think about supporting it, but until then reporting any value is
tantamount to lying.
>
>> 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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