Re: [PATCH v1 0/2] vfio/pci: expose device's PASID capability to VMs

Lu, Baolu
2020-03-31T07:08:40Z

On 2020/3/31 14:35, Tian, Kevin wrote:
>> From: Liu, Yi L<yi.l.liu@intel.com>
>> Sent: Sunday, March 22, 2020 8:33 PM
>>
>> From: Liu Yi L<yi.l.liu@intel.com>
>>
>> Shared Virtual Addressing (SVA), a.k.a, Shared Virtual Memory (SVM) on
>> Intel platforms allows address space sharing between device DMA and
>> applications. SVA can reduce programming complexity and enhance security.
>>
>> To enable SVA, device needs to have PASID capability, which is a key
>> capability for SVA. This patchset exposes the device's PASID capability
>> to guest instead of hiding it from guest.
>>
>> The second patch emulates PASID capability for VFs (Virtual Function) since
>> VFs don't implement such capability per PCIe spec. This patch emulates such
>> capability and expose to VM if the capability is enabled in PF (Physical
>> Function).
>>
>> However, there is an open for PASID emulation. If PF driver disables PASID
>> capability at runtime, then it may be an issue. e.g. PF should not disable
>> PASID capability if there is guest using this capability on any VF related
>> to this PF. To solve it, may need to introduce a generic communication
>> framework between vfio-pci driver and PF drivers. Please feel free to give
>> your suggestions on it.
> I'm not sure how this is addressed on bate metal today, i.e. between normal
> kernel PF and VF drivers. I look at pasid enable/disable code in intel-iommu.c.
> There is no check on PF/VF dependency so far. The cap is toggled when
> attaching/detaching the PF to its domain. Let's see how IOMMU guys
> respond, and if there is a way for VF driver to block PF driver from disabling
> the pasid cap when it's being actively used by VF driver, then we may
> leverage the same trick in VFIO when emulation is provided to guest.

IOMMU subsystem doesn't expose any APIs for pasid enabling/disabling.
The PCI subsystem does. It handles VF/PF like below.

/**
* pci_enable_pasid - Enable the PASID capability
* @pdev: PCI device structure
* @features: Features to enable
*
* Returns 0 on success, negative value on error. This function checks
* whether the features are actually supported by the device and returns
* an error if not.
*/
int pci_enable_pasid(struct pci_dev *pdev, int features)
{
u16 control, supported;
int pasid = pdev->pasid_cap;

/*
* VFs must not implement the PASID Capability, but if a PF
* supports PASID, its VFs share the PF PASID configuration.
*/
if (pdev->is_virtfn) {
if (pci_physfn(pdev)->pasid_enabled)
return 0;
return -EINVAL;
}

/**
* pci_disable_pasid - Disable the PASID capability
* @pdev: PCI device structure
*/
void pci_disable_pasid(struct pci_dev *pdev)
{
u16 control = 0;
int pasid = pdev->pasid_cap;

/* VFs share the PF PASID configuration */
if (pdev->is_virtfn)
return;

It doesn't block disabling PASID on PF even VFs are possibly using it.

Best regards,
baolu