Re: [PATCH v6 08/11] eal: pci: introduce RTE_KDRV_VFIO_NOIOMMUi driver mode

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-01-26 19:35, Santosh Shukla:
> On Tue, Jan 26, 2016 at 6:30 PM, Thomas Monjalon
> <thomas.monjalon@6wind.com> wrote:
> > 2016-01-26 15:56, Santosh Shukla:
> >> In my observation, currently virtio work for vfio-noiommu, that's why
> >> said drv->kdrv need to know vfio mode.
> >
> > It is your observation. It may change in near future.
> 
> so that mean till then, virtio support for non-x86 arch has to wait?

No, absolutely not. virtio for non-x86 is welcome.

> We have working model with vfio-noiommu, don't you think it make sense
> to let vfio_noiommu implementation exist and later in-case
> virtio+iommu gets mainline then switch to vfio __mode__ agnostic
> approach. And for that All it takes to replace __noiommu suffix with
> default.

I'm just saying you should not touch the enum rte_kernel_driver.
RTE_KDRV_VFIO is a driver.
RTE_KDRV_VFIO_NOIOMMU is a mode.
As the VFIO API is the same in both modes, there is no reason to
distinguish them at this level.
Your patch adds the NOIOMMU case everywhere:
        case RTE_KDRV_VFIO:
+       case RTE_KDRV_VFIO_NOIOMMU:

I'll stop commenting here to let others give their opinion.

[...]
> >> with vfio+iommu; binding virtio pci device to vfio-pci driver fail;
> >> giving below error:
> >> [   53.053464] VFIO - User Level meta-driver version: 0.3
> >> [   73.077805] vfio-pci: probe of 0000:00:03.0 failed with error -22
> >> [   73.077852] vfio-pci: probe of 0000:00:03.0 failed with error -22
> >>
> >> vfio_pci_probe() --> vfio_iommu_group_get() --> iommu_group_get()
> >> fails: iommu doesn't have group for virtio pci device.
> >
> > Yes it fails when binding.
> > So the later check in the virtio PMD is useless.
> 
> Which check?

The check for VFIO noiommu only:
-       if (dev->kdrv == RTE_KDRV_VFIO)
+       if (dev->kdrv == RTE_KDRV_VFIO_NOIOMMU)

[...]
> > Furthermore restricting virtio to no-iommu mode doesn't bring
> > any improvement.
> 
> We're not __restricting__, as soon as virtio+iommu gets working state,
> we'll simply replace __noiommu with default. Then its upto user to try
> out virtio with vfio default or vfio_noiommu.

Yes it's up to user.
So your code should be
	if (dev->kdrv == RTE_KDRV_VFIO)

> > That's why I suggest to keep the initial semantic of kdrv and
> > not pollute it with VFIO modes.
> 
> I am okay to live with default and forget suffix __noiommu but there
> are implementation problem which was discussed in other thread
> - Virtio pmd driver should avoid interface parsing i.e.
> virtio_resource_init_uio/vfio() etc.. For vfio case - We could easily
> get rid of by moving /sys parsing to pci_eal layer, Right? If so then
> virtio currently works with vfio-noiommu, it make sense to me that
> pci_eal layer does parsing for pmd driver before that pmd driver get
> initialized.

Please reword. What is the problem?

> - Another case could be: iommu-less-pmd-driver. eal layer to do
> parsing before updating drv->kdrv.

[...]
> >> >> > If a check is needed, I would prefer using your function
> >> >> > pci_vfio_is_noiommu() and remove driver modes from struct rte_kernel_driver.
> >> >>
> >> >> I don't think calling pci_vfio_no_iommu() inside
> >> >> virtio_reg_rd/wr_1/2/3() would be a good idea.
> >> >
> >> > Why? The value may be cached in the priv properties.
> >> >
> >> pci_vfio_is_noiommu() parses /sys for
> >> - enable_noiommu param
> >> - attached driver name is vfio-noiommu or not.
> >>
> >> It does file operation for that, I meant to say that calling this api
> >> within register_rd/wr function is not correct. It would be better if
> >> those low level register_rd/wr api only checks driver_types.
> >
> > Yes, that's why I said the return of pci_vfio_is_noiommu() may be cached
> > to keep efficiency.
> 
> I am not convinced though, Still find pmd driver checking driver_types
> using drv->kdrv is better approach than introducing a new global
> variable which may look something like;

Not a global variable. A function in EAL layer. A variable in PMD priv.

> At pci_eal layer ----
> bool vfio_mode;
> vfio_mode = pci_vfio_is_noiommu();
> 
> At virtio pmd driver layer ----
> Checking value at vfio_mode variable before doing virtio_rd/wr for
> vfio interface.
> 
> Instead virtio pmd driver doing
> 
> virtio_reg_rd/wr_1/2/4()
> {
> if (drv->kdrv == VFIO)
>       do pread()/pwrite()
> else
>       in()/out()
> }
> 
> is better approach.
> 
> Let me know if you still think former is better than latter then I'll
> send patch revision right-away.