Re: [dpdk-dev] Having troubles binding an SR-IOV VF to uio_pci_generic on Amazon instance

[Avi Kivity <avi@scylladb.com>]

On 09/30/2015 05:39 PM, Michael S. Tsirkin wrote:
> On Wed, Sep 30, 2015 at 04:05:40PM +0300, Avi Kivity wrote:
>>
>> On 09/30/2015 03:27 PM, Michael S. Tsirkin wrote:
>>> On Wed, Sep 30, 2015 at 03:16:04PM +0300, Vlad Zolotarov wrote:
>>>> On 09/30/15 15:03, Michael S. Tsirkin wrote:
>>>>> On Wed, Sep 30, 2015 at 02:53:19PM +0300, Vlad Zolotarov wrote:
>>>>>> On 09/30/15 14:41, Michael S. Tsirkin wrote:
>>>>>>> On Wed, Sep 30, 2015 at 02:26:01PM +0300, Vlad Zolotarov wrote:
>>>>>>>> The whole idea is to bypass kernel. Especially for networking...
>>>>>>> ... on dumb hardware that doesn't support doing that securely.
>>>>>> On a very capable HW that supports whatever security requirements needed
>>>>>> (e.g. 82599 Intel's SR-IOV VF devices).
>>>>> Network card type is irrelevant as long as you do not have an IOMMU,
>>>>> otherwise you would just use e.g. VFIO.
>>>> Sorry, but I don't follow your logic here - Amazon EC2 environment is a
>>>> example where there *is* iommu but it's not virtualized
>>>> and thus VFIO is
>>>> useless and there is an option to use directly assigned SR-IOV networking
>>>> device there where using the kernel drivers impose a performance impact
>>>> compared to user space UIO-based user space kernel bypass mode of usage. How
>>>> is it irrelevant? Could u, pls, clarify your point?
>>>>
>>> So it's not even dumb hardware, it's another piece of software
>>> that forces an "all or nothing" approach where either
>>> device has access to all VM memory, or none.
>>> And this, unfortunately, leaves you with no secure way to
>>> allow userspace drivers.
>> Some setups don't need security (they are single-user, single application).
>> But do need a lot of performance (like 5X-10X performance).  An example is
>> OpenVSwitch, security doesn't help it at all and if you force it to use the
>> kernel drivers you cripple it.
> We'd have to see there are actual users that need this.  So far, dpdk
> seems like the only one,

dpdk is a whole class if users.  It's not a specific application.

>   and it wants to use UIO for slow path stuff
> like polling link status.  Why this needs kernel bypass support, I don't
> know.  I asked, and got no answer.

First, it's more than link status.  dpdk also has an interrupt mode, 
which applications can fall back to when when the load is light in order 
to save power (and in order not to get support calls about 100% cpu when 
idle).

Even for link status, you don't want to poll for that, because accessing 
device registers is expensive.  An interrupt is the best approach for 
rare events like link changed.

>
>> Also, I'm root.  I can do anything I like, including loading a patched
>> pci_uio_generic.  You're not providing _any_ security, you're simply making
>> life harder for users.
> Maybe that's true on your system. But I guess you know that's not true
> for everyone, not in 2015.

Why is it not true?  if I'm root, I can do anything I like to my system, 
and everyone is root in 2015.  I can access the BARs directly and 
program DMA, how am I more secure by uio not allowing me to setup msix?

Non-root users are already secured by their inability to load the 
module, and by the device permissions.

>
>>> So it makes even less sense to add insecure work-arounds in the kernel.
>>> It seems quite likely that by the time the new kernel reaches
>>> production X years from now, EC2 will have a virtual iommu.
>> I can adopt a new kernel tomorrow.  I have no influence on EC2.
>>
>>
> Xen grant tables sound like they could be the right interface
> for EC2.  google search for "grant tables iommu" immediately gives me:
> http://lists.xenproject.org/archives/html/xen-devel/2014-04/msg00963.html
> Maybe latest Xen is already doing the right thing, and it's just the
> question of making VFIO use that.
>

grant tables only work for virtual devices, not physical devices.