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Subject: Re: [PATCH v2 1/2] KVM: Start populating /sys/hypervisor with KVM
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To:     "Sironi, Filippo" <sironi@amazon.de>
CC:     LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        "cohuck@redhat.com" <cohuck@redhat.com>,
        "konrad.wilk@oracle.com" <konrad.wilk@oracle.com>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        "Marc Zyngier" <Marc.Zyngier@arm.com>,
        Christoffer Dall <christoffer.dall@linaro.org>
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From:   Alexander Graf <graf@amazon.com>
Message-ID: <6b3dadf9-6240-6440-b784-50bec605bf2c@amazon.com>
Date:   Fri, 31 May 2019 11:06:13 +0200
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On 17.05.19 17:41, Sironi, Filippo wrote:
>> On 16. May 2019, at 15:50, Graf, Alexander <graf@amazon.com> 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@amazon.de>
>> 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);