> From: Alex Williamson > Sent: Monday, March 9, 2020 8:46 AM > > On Sat, 7 Mar 2020 01:35:23 +0000 > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > > > From: Alex Williamson > > > Sent: Saturday, March 7, 2020 6:18 AM > > > > > > On Fri, 6 Mar 2020 07:57:19 +0000 > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > > > > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > > > Sent: Friday, March 6, 2020 2:23 AM > > > > > > > > > > On Tue, 25 Feb 2020 03:08:00 +0000 > > > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > > > > > > > > > > > From: Alex Williamson > > > > > > > Sent: Thursday, February 20, 2020 2:54 AM > > > > > > > > > > > > > > With the VF Token interface we can now expect that a vfio > userspace > > > > > > > driver must be in collaboration with the PF driver, an unwitting > > > > > > > userspace driver will not be able to get past the GET_DEVICE_FD > step > > > > > > > in accessing the device. We can now move on to actually allowing > > > > > > > SR-IOV to be enabled by vfio-pci on the PF. Support for this is not > > > > > > > enabled by default in this commit, but it does provide a module > > > option > > > > > > > for this to be enabled (enable_sriov=1). Enabling VFs is rather > > > > > > > straightforward, except we don't want to risk that a VF might get > > > > > > > autoprobed and bound to other drivers, so a bus notifier is used > to > > > > > > > "capture" VFs to vfio-pci using the driver_override support. We > > > > > > > assume any later action to bind the device to other drivers is > > > > > > > condoned by the system admin and allow it with a log warning. > > > > > > > > > > > > > > vfio-pci will disable SR-IOV on a PF before releasing the device, > > > > > > > allowing a VF driver to be assured other drivers cannot take over > the > > > > > > > PF and that any other userspace driver must know the shared VF > > > token. > > > > > > > This support also does not provide a mechanism for the PF > userspace > > > > > > > driver itself to manipulate SR-IOV through the vfio API. With this > > > > > > > patch SR-IOV can only be enabled via the host sysfs interface and > the > > > > > > > PF driver user cannot create or remove VFs. > > > > > > > > > > > > I'm not sure how many devices can be properly configured simply > > > > > > with pci_enable_sriov. It is not unusual to require PF driver prepare > > > > > > something before turning PCI SR-IOV capability. If you look kernel > > > > > > PF drivers, there are only two using generic pci_sriov_configure_ > > > > > > simple (simple wrapper like pci_enable_sriov), while most others > > > > > > implementing their own callback. However vfio itself has no idea > > > > > > thus I'm not sure how an user knows whether using this option can > > > > > > actually meet his purpose. I may miss something here, possibly > > > > > > using DPDK as an example will make it clearer. > > > > > > > > > > There is still the entire vfio userspace driver interface. Imagine for > > > > > example that QEMU emulates the SR-IOV capability and makes a call > out > > > > > to libvirt (or maybe runs with privs for the PF SR-IOV sysfs attribs) > > > > > when the guest enables SR-IOV. Can't we assume that any PF specific > > > > > support can still be performed in the userspace/guest driver, leaving > > > > > us with a very simple and generic sriov_configure callback in vfio-pci? > > > > > > > > Makes sense. One concern, though, is how an user could be warned > > > > if he inadvertently uses sysfs to enable SR-IOV on a vfio device whose > > > > userspace driver is incapable of handling it. Note any VFIO device, > > > > if SR-IOV capable, will allow user to do so once the module option is > > > > turned on and the callback is registered. I felt such uncertainty can be > > > > contained by toggling SR-IOV through a vfio api, but from your > description > > > > obviously it is what you want to avoid. Is it due to the sequence reason, > > > > e.g. that SR-IOV must be enabled before userspace PF driver sets the > > > > token? > > > > > > As in my other reply, enabling SR-IOV via a vfio API suggests that > > > we're not only granting the user owning the PF device access to the > > > device itself, but also the ability to create and remove subordinate > > > devices on the host. That implies an extended degree of trust in the > > > user beyond the PF device itself and raises questions about whether a > > > user who is allowed to create VF devices should automatically be > > > granted access to those VF devices, what the mechanism would be for > > > that, and how we might re-assign those devices to other users, > > > potentially including host kernel usage. What I'm proposing here > > > doesn't preclude some future extension in that direction, but instead > > > tries to simplify a first step towards enabling SR-IOV by leaving the > > > SR-IOV enablement and VF assignment in the realm of a privileged system > > > entity. > > > > the intention is clear to me now. > > > > > > > > So, what I think you're suggesting here is that we should restrict > > > vfio_pci_sriov_configure() to reject enabling SR-IOV until a user > > > driver has configured a VF token. That requires both that the > > > userspace driver has initialized to this point before SR-IOV can be > > > enabled and that we would be forced to define a termination point for > > > the user set VF token. Logically, this would need to be when the > > > userspace driver exits or closes the PF device, which implies that we > > > need to disable SR-IOV on the PF at this point, or we're left in an > > > inconsistent state where VFs are enabled but cannot be disabled because > > > we don't have a valid VF token. Now we're back to nearly a state where > > > the user has control of not creating devices on the host, but removing > > > them by closing the device, which will necessarily require that any VF > > > driver release the device, whether userspace or kernel. > > > > > > I'm not sure what we're gaining by doing this though. I agree that > > > there will be users that enable SR-IOV on a PF and then try to, for > > > example, assign the PF and all the VFs to a VM. The VFs will fail due > > > to lacking VF token support, unless they've patch QEMU with my test > > > code, but depending on the PF driver in the guest, it may, or more > > > likely won't work. But don't you think the VFs and probably PF not > > > working is a sufficient clue that the configuration is invalid? OTOH, > > > from what I've heard of the device in the ID table of the pci-pf-stub > > > driver, they might very well be able to work with both PF and VFs in > > > QEMU using only my test code to set the VF token. > > > > > > Therefore, I'm afraid what you're asking for here is to impose a usage > > > restriction as a sanity test, when we don't really know what might be > > > sane for this particular piece of hardware or use case. There are > > > infinite ways that a vfio based userspace driver can fail to configure > > > their hardware and make it work correctly, many of them are device > > > specific. Isn't this just one of those cases? Thanks, > > > > > > > what you said all makes sense. so I withdraw the idea of manipulating > > SR-IOV through vfio ioctl. However I still feel that simply registering > > sriov_configuration callback by vfio-pci somehow violates the typical > > expectation of the sysfs interface. Before this patch, the success return > > of writing non-zero value to numvfs implies VFs are in sane state and > > functionally ready for immediate use. However now the behavior of > > success return becomes undefined for vfio devices, since even vfio-pci > > itself doesn't know whether VFs are functional for a random device > > (may know some if carrying the same device IDs from pci-pf-stub). It > > simply relies on the privileged entity who knows exactly the implication > > of such write, while there is no way to warn inadvertent users which > > to me is not a good design from kernel API p.o.v. Of course we may > > document such restriction and the driver_override may also be an > > indirect way to warn such user if he wants to use VFs for other purpose. > > But it is still less elegant than reporting it in the first place. Maybe > > what we really require is a new sysfs attribute purely for enabling > > PCI SR-IOV capability, which doesn't imply making VFs actually > > functional as did through the existing numvfs? > > I don't read the same guarantee into the sysfs SR-IOV interface. If > such a guarantee exists, it's already broken by pci-pf-stub, which like > vfio-pci allows dynamic IDs and driver_override to bind to any PF device > allowing the ability to create (potentially) non-functional VFs. I I don't know whether others raised the similar concern and how it was addressed for pci-pf-stub before. Many places describe numvfs as the preferred interface to enable/disable VFs while 'enable' just reads functional to me. > think it would be a really bad decision to fork a new sysfs interface > for this. I've already made SR-IOV support in vfio-pci an opt-in via a > module option, would it ease your concerns if I elaborate in the text > for the option that enabling SR-IOV may depend on support provided by a > vfio-pci userspace driver? Sure. > > I think that without absolutely knowing that an operation is incorrect, > we're just generating noise and confusion by triggering warnings or > developing alternate interfaces. Unfortunately, we have no generic > means of knowing that an operation is incorrect, so I assume the best. > Thanks, > > Alex
