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

[Santosh Shukla <sshukla@mvista.com>]

On Tue, Jan 26, 2016 at 7:58 PM, Thomas Monjalon
<thomas.monjalon@6wind.com> wrote:
> 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)
>

Right,

>> > 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.
>

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/

>> 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.
>
>