Checking bhyve supported features (sysctls)

Matt Churchyard matt.churchyard at
Thu Aug 16 20:43:56 UTC 2018

On 16 Aug 2018, at 19:55, Marcelo Araujo <araujobsdport at<mailto:araujobsdport at>> wrote:

2018-08-17 0:53 GMT+08:00 Allan Jude <allanjude at<mailto:allanjude at>>:
On August 16, 2018 5:28:05 PM GMT+01:00, "Rodney W. Grimes" <freebsd-rwg at<mailto:freebsd-rwg at>> wrote:
>> Text manually wrapped to 80, any broken quoting is my fault - rwg
>> > > Hello,
>> > >
>> > > I'm looking for better ways to check for  bhyve support /
>> > > features without trying to scan through dmesg output.
>> >
>> > >Yes, it would be very good to remove that, as it usually tries
>> > >to grep a non-existent file /var/run/dmesg.boot that is not
>> > >created until after vm_bhyve has been called from
>> > >when you have things set to autostartup >in /etc/rc.conf
>> >
>> >
>> > >
>> > > I notice that the following 2 sysctl's appear to be set to 1 as
>> > > as the vmm module is loaded
>> > >
>> > > hw.vmm.vmx.initialized: 1
>> > > hw.vmm.vmx.cap.unrestricted_guest: 1
>> > >
>> > > Will these be available on both Intel & AMD processors as a way
>> > > to determine if the module has loaded successfully and can run
>> > >
>> > > I also see the below sysctl related to iommu.
>> > >
>> > > hw.vmm.iommu.initialized
>> > >
>> > > Again, will this be set to 1 as soon as the module is loaded if
>> > > iommu is supported, or only when it is used?
>> > > There also seems to be a vmm.amdvi.enable sysctl.
>> > > Would both these need checking or is vmm.iommu enough to
>> > > determine support on any processor.
>> >
>> > >Probalby the safest way for a shell script to decide if bhyve is
>> > >up and running is to stat /dev/vmm, if that exists then the
>> > >have loaded and initialized and bhyve should be ready to process
>> >
>> > Hmm, I don't get /dev/vmm unless I actually have running guests.
>> I'll investigate that, I was pretty sure that you should get this
>> as soon as the vmm.ko module is finished initialzing, but you might
>> be right in that it takes a first vm to cause its creation.
>> Confirmed, /dev/vmm does not exist until the first vm
>> is created.
>> >
>> > >sysctl's mentiond above would be a poor way to make this
>> >
>> > It would be nice if sysctls were better documented.
>> Agreed.
>> > If vmx.initialized is set once vmm has successfully loaded, I can't
>see a better way of checking for bhyve support (assuming it's not Intel
>specific). This entry definitely exists and is set to 0 if you load the
>module on a non-supported system, and set to 1 as soon as vmm loads on
>my Intel test system.
>> Given its undocumented status you would be relying on an
>> undocumented feature that could change in either name or
>> behavior, and that is not desirable.
>> Let me see if I can come up with something else.
>I looked at the code for bhyvectl, bhyveload and
>byhve.  They do not actually try to decide if vmm
>is supported or not, they simply process the error
>from a vm_create() or vm_open() call and exit
>with an error code if they can not handle it
>(some of the code can handle a vm_create failure
>if infact we are trying to create a vm that
>already exists).
>If you want to maintain full compatibility a similiar
>stratergy may be in order.
>Why is it that vm-bhyve specifically needs to know
>if the kernel has vmm support or not?
>Cant it just be written to handle the errors returned
>if the supported functions do not exist?

I think the question vm-bhyve wants to answer is: does the CPU have the required features to run a multicore VM.

These or similar sysctls do seem to be the correct way to communicate that support.

You are correct!

The question in case as I understood was about CPU feature supported, actually vmm(8) knows all this information! Some examples such like CPU with VMX unrestricted mode support (UG) that is necessary for guest VMs running with multiple vCPU or like VT-d necessary for PCI device passthrough.

I have a patch that exposes a sysctl saying what bhyve(8) is capable to run, however it needs to be polished a bit more to be more informative.
I think for third part software like vm-bhyve these information are crucial as these software can get advantage of these information prior to run a certain set that will end up in a fail because of a partial CPU support.


As mentioned in my first email, it does seem like some of these exist already in the way of
vmm.vmx.cap.* sysctls.

We could look at bhyve output and try to process that, but that seems more messy if there
are sysctls that expose support, especially as the vmm module does seem to know what
features the cpu/hardware supports. vm-bhyve has already forked into the background
by the time bhyve runs so can't easily provide feedback to the caller other than through
the log file.

We do also try and take action in some cases, such as reducing cpu count to 1 if UG support
isn't found, rather than just having bhyve fail. (Of course you could argue we should just
exit with an error and let the user decide if they want to drop the cpu count to 1.

We could just do nothing, let bhyve run and if it falls over people can use debug mode and
see the bhyve output themselves in the log. Just seems useful to be able to tell users
that their hardware doesn't support the features they are trying to use up front.

It still seems to me that vmx.initialised is a reasonable indicator that vmm has loaded
without issue, but it would be useful to have some documented way of checking
exactly what virt features the system supports, without just running something and
seeing if it falls over.


Allan Jude
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Marcelo Araujo            (__)
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