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

On Tue, Jan 26, 2016 at 7:58 PM, Thomas Monjalon
[off-list ref] wrote:

2016-01-26 19:35, Santosh Shukla:

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On Tue, Jan 26, 2016 at 6:30 PM, Thomas Monjalon
[off-list ref] wrote:

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2016-01-26 15:56, Santosh Shukla:

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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.

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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.

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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)

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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)

Right,

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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?

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

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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.

If we agreed to use condition (drv->kdrv == RTE_KDRV_VFIO);
then resource parsing for vfio {including vfio and vfio_noiommu both
case} is enforced in virtio pmd driver layer and that is contradicting
to what we agreed earlier in this[1] thread. Also we don't need a
function in EAL layer or a variable in PMD priv. Perhaps a private
function in virtio pmd which does parsing for vfio interface.

Thoughts?

[1] http://dpdk.org/dev/patchwork/patch/9862/

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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.