Re: [PATCH v2 0/7] vfio/pci: SR-IOV support

Jason Wang
2020-02-25T06:09:27Z

On 2020/2/25 上午10:33, Tian, Kevin wrote:
>> From: Alex Williamson
>> Sent: Thursday, February 20, 2020 2:54 AM
>>
>> Changes since v1 are primarily to patch 3/7 where the commit log is
>> rewritten, along with option parsing and failure logging based on
>> upstream discussions. The primary user visible difference is that
>> option parsing is now much more strict. If a vf_token option is
>> provided that cannot be used, we generate an error. As a result of
>> this, opening a PF with a vf_token option will serve as a mechanism of
>> setting the vf_token. This seems like a more user friendly API than
>> the alternative of sometimes requiring the option (VFs in use) and
>> sometimes rejecting it, and upholds our desire that the option is
>> always either used or rejected.
>>
>> This also means that the VFIO_DEVICE_FEATURE ioctl is not the only
>> means of setting the VF token, which might call into question whether
>> we absolutely need this new ioctl. Currently I'm keeping it because I
>> can imagine use cases, for example if a hypervisor were to support
>> SR-IOV, the PF device might be opened without consideration for a VF
>> token and we'd require the hypservisor to close and re-open the PF in
>> order to set a known VF token, which is impractical.
>>
>> Series overview (same as provided with v1):
> Thanks for doing this!
>
>> The synopsis of this series is that we have an ongoing desire to drive
>> PCIe SR-IOV PFs from userspace with VFIO. There's an immediate need
>> for this with DPDK drivers and potentially interesting future use
> Can you provide a link to the DPDK discussion?
>
>> cases in virtualization. We've been reluctant to add this support
>> previously due to the dependency and trust relationship between the
>> VF device and PF driver. Minimally the PF driver can induce a denial
>> of service to the VF, but depending on the specific implementation,
>> the PF driver might also be responsible for moving data between VFs
>> or have direct access to the state of the VF, including data or state
>> otherwise private to the VF or VF driver.
> Just a loud thinking. While the motivation of VF token sounds reasonable
> to me, I'm curious why the same concern is not raised in other usages.
> For example, there is no such design in virtio framework, where the
> virtio device could also be restarted, putting in separate process (vhost-user),
> and even in separate VM (virtio-vhost-user), etc.

AFAIK, the restart could only be triggered by either VM or qemu. But
yes, the datapath could be offloaded.

But I'm not sure introducing another dedicated mechanism is better than
using the exist generic POSIX mechanism to make sure the connection
(AF_UINX) is secure.

> Of course the para-
> virtualized attribute of virtio implies some degree of trust, but as you
> mentioned many SR-IOV implementations support VF->PF communication
> which also implies some level of trust. It's perfectly fine if VFIO just tries
> to do better than other sub-systems, but knowing how other people
> tackle the similar problem may make the whole picture clearer. 😊
>
> +Jason.

I'm not quite sure e.g allowing userspace PF driver with kernel VF
driver would not break the assumption of kernel security model. At least
we should forbid a unprivileged PF driver running in userspace.

Thanks