Re: [PATCH v2 1/2] KVM: Start populating /sys/hypervisor with KVM entries

[Alexander Graf]

On 17.05.19 17:41, Sironi, Filippo wrote:
> > On 16. May 2019, at 15:50, Graf, Alexander <graf@xxxxxxxxxx> wrote:
> >
> > On 14.05.19 08:16, Filippo Sironi wrote:
> > > Start populating /sys/hypervisor with KVM entries when we're running on
> > > KVM. This is to replicate functionality that's available when we're
> > > running on Xen.
> > >
> > > Start with /sys/hypervisor/uuid, which users prefer over
> > > /sys/devices/virtual/dmi/id/product_uuid as a way to recognize a virtual
> > > machine, since it's also available when running on Xen HVM and on Xen PV
> > > and, on top of that doesn't require root privileges by default.
> > > Let's create arch-specific hooks so that different architectures can
> > > provide different implementations.
> > >
> > > Signed-off-by: Filippo Sironi <sironi@xxxxxxxxx>
> > I think this needs something akin to
> >
> >   https://www.kernel.org/doc/Documentation/ABI/stable/sysfs-hypervisor-xen
> >
> > to document which files are available.
> >
> > > ---
> > > v2:
> > > * move the retrieval of the VM UUID out of uuid_show and into
> > >   kvm_para_get_uuid, which is a weak function that can be overwritten
> > >
> > > drivers/Kconfig              |  2 ++
> > > drivers/Makefile             |  2 ++
> > > drivers/kvm/Kconfig          | 14 ++++++++++++++
> > > drivers/kvm/Makefile         |  1 +
> > > drivers/kvm/sys-hypervisor.c | 30 ++++++++++++++++++++++++++++++
> > > 5 files changed, 49 insertions(+)
> > > create mode 100644 drivers/kvm/Kconfig
> > > create mode 100644 drivers/kvm/Makefile
> > > create mode 100644 drivers/kvm/sys-hypervisor.c
> > [...]
> >
> > > +
> > > +__weak const char *kvm_para_get_uuid(void)
> > > +{
> > > +	return NULL;
> > > +}
> > > +
> > > +static ssize_t uuid_show(struct kobject *obj,
> > > +			 struct kobj_attribute *attr,
> > > +			 char *buf)
> > > +{
> > > +	const char *uuid = kvm_para_get_uuid();
> > > +	return sprintf(buf, "%s\n", uuid);
> > The usual return value for the Xen /sys/hypervisor interface is
> > "<denied>". Wouldn't it make sense to follow that pattern for the KVM
> > one too? Currently, if we can not determine the UUID this will just
> > return (null).
> >
> > Otherwise, looks good to me. Are you aware of any other files we should
> > provide? Also, is there any reason not to implement ARM as well while at it?
> >
> > Alex
> This originated from a customer request that was using /sys/hypervisor/uuid.
> My guess is that we would want to expose "type" and "version" moving
> forward and that's when we hypervisor hooks will be useful on top
> of arch hooks.
>
> On a different note, any idea how to check whether the OS is running
> virtualized on KVM on ARM and ARM64?  kvm_para_available() isn't an


Yeah, ARM doesn't have any KVM PV FWIW. I also can't find any explicit 
hint passed into guests that we are indeed running in KVM. The closest 
thing I can see is the SMBIOS product identifier in QEMU which gets 
patched to "KVM Virtual Machine". Maybe we'll have to do with that for 
the sake of backwards compatibility ...


> option and the same is true for S390 where kvm_para_available()
> always returns true and it would even if a KVM enabled kernel would
> be running on bare metal.


For s390, you can figure the topology out using the sthyi instruction. 
I'm not sure if there is a nice in-kernel API to leverage that though. 
In fact, kvm_para_available() probably should check sthyi output to 
determine whether we really can use it, no? Christian?


Alex


> I think we will need another arch hook to call a function that says
> whether the OS is running virtualized on KVM.
>
> > > +}
> > > +
> > > +static struct kobj_attribute uuid = __ATTR_RO(uuid);
> > > +
> > > +static int __init uuid_init(void)
> > > +{
> > > +	if (!kvm_para_available())
> > > +		return 0;
> > > +	return sysfs_create_file(hypervisor_kobj, &uuid.attr);
> > > +}
> > > +
> > > +device_initcall(uuid_init);