Re: [PATCH v4 4/6] host/rootfs: Sandbox Cloud Hypervisor

Alyssa Ross
2025-12-06T17:46:45Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> On 12/4/25 09:35, Alyssa Ross wrote:
>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>
>>> It only needs access to a small number of resources. Unfortunately, it
>>> needs access to /dev/vfio right now. This should be fixed by using file
>>> descriptor passing instead.
>>>
>>> Furthermore, Cloud Hypervisor needs to be able to lock memory. Running
>>> in a user namespace prevents it from having CAP_IPC_LOCK. Therefore, it
>>> is necessary to increase RLIMIT_MLOCK before running Cloud Hypervisor.
>>>
>>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>>> ---
>>> .../image/etc/udev/rules.d/99-spectrum.rules | 3 ++
>>> host/rootfs/image/usr/bin/run-vmm | 33 +++++++++++++++++++++-
>>> 2 files changed, 35 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/host/rootfs/image/etc/udev/rules.d/99-spectrum.rules b/host/rootfs/image/etc/udev/rules.d/99-spectrum.rules
>>> index 337bbe47dbbc6f3828722d8244f2689a39f3090f..de0f682aa40f8481dc3c25a90c695e2326536316 100644
>>> --- a/host/rootfs/image/etc/udev/rules.d/99-spectrum.rules
>>> +++ b/host/rootfs/image/etc/udev/rules.d/99-spectrum.rules
>>> @@ -3,3 +3,6 @@
>>>
>>> # systemd-udevd unsets PATH, so fix that.
>>> ACTION!="remove", ENV{PCI_CLASS}=="2????", RUN+="/usr/bin/env PATH=/usr/bin /usr/libexec/net-add"
>>> +
>>> +# make /dev/kvm world-accessible
>>> +KERNEL=="kvm", MODE="0666"
>>> diff --git a/host/rootfs/image/usr/bin/run-vmm b/host/rootfs/image/usr/bin/run-vmm
>>> index ba8b59c2677408acdd01c2eda3cf2dd60992d881..24c3d607bfcf6fea6196b61d2941141486d33fd6 100755
>>> --- a/host/rootfs/image/usr/bin/run-vmm
>>> +++ b/host/rootfs/image/usr/bin/run-vmm
>>> @@ -52,5 +52,36 @@ unexport !
>>> fdmove -c 3 0
>>> redirfd -r 0 /dev/null
>>>
>>> +s6-softlimit -H -l 18446744073709551615
>>
>> The s6-softlimit documentation says that hard limits should generally
>> only be set once, at boot, and that's what we now do for PipeWire in
>> img/app. Is the idea here that it would be undesirable to incraese the
>> hard limit for all processes, so only do it for Cloud Hypervisor?
>
> s6-softlimit -H also increases the soft limit. Allowing every
> process on the system to lock an unlimited amount of memory doesn't
> seem ideal. For interactive logins, soft limits will be set via PAM,
> but Spectrum doesn't use PAM yet. This keeps the change localized,
> rather than having to bump the hard limit everywhere and then undo
> the change elsewhere.

I wonder why the documentation says that, then. I suppose that's
something I should take up with skarnet rather than you?

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