[dpdk-dev] [EXT] Re: [PATCH v6 4/4] kernel/linux/kni: add IOVA support in kni module

[Vamsi Krishna Attunuru]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit at intel.com>
> Sent: Friday, July 12, 2019 4:40 PM
> To: Vamsi Krishna Attunuru <vattunuru at marvell.com>; dev at dpdk.org
> Cc: olivier.matz at 6wind.com; arybchenko at solarflare.com; Kiran Kumar
> Kokkilagadda <kirankumark at marvell.com>
> Subject: Re: [EXT] Re: [PATCH v6 4/4] kernel/linux/kni: add IOVA support in
> kni module
> 
> On 7/12/2019 11:38 AM, Vamsi Krishna Attunuru wrote:
> >
> >
> >
> > ----------------------------------------------------------------------
> > ----------
> > *From:* Ferruh Yigit <ferruh.yigit at intel.com>
> > *Sent:* Thursday, July 11, 2019 10:00 PM
> > *To:* Vamsi Krishna Attunuru; dev at dpdk.org
> > *Cc:* olivier.matz at 6wind.com; arybchenko at solarflare.com; Kiran Kumar
> > Kokkilagadda
> > *Subject:* [EXT] Re: [PATCH v6 4/4] kernel/linux/kni: add IOVA support
> > in kni module
> >
> > External Email
> >
> > ----------------------------------------------------------------------
> > On 6/25/2019 4:57 AM, vattunuru at marvell.com wrote:
> >> From: Kiran Kumar K <kirankumark at marvell.com>
> >>
> >> Patch adds support for kernel module to work in IOVA = VA mode, the
> >> idea is to get physical address from iova address using
> >> iommu_iova_to_phys API and later use phys_to_virt API to convert the
> >> physical address to kernel virtual address.
> >>
> >> When compared with IOVA = PA mode, there is no performance drop
> with
> >> this approach.
> >>
> >> This approach does not work with the kernel versions less than 4.4.0
> >> because of API compatibility issues.
> >>
> >> Signed-off-by: Kiran Kumar K <kirankumark at marvell.com>
> >> Signed-off-by: Vamsi Attunuru <vattunuru at marvell.com>
> >
> > <...>
> >
> >> @@ -351,15 +354,56 @@ kni_ioctl_create(struct net *net, uint32_t
> >>ioctl_num,
> >>        strncpy(kni->name, dev_info.name, RTE_KNI_NAMESIZE);
> >>
> >>        /* Translate user space info into kernel space info */
> >> -     kni->tx_q = phys_to_virt(dev_info.tx_phys);
> >> -     kni->rx_q = phys_to_virt(dev_info.rx_phys);
> >> -     kni->alloc_q = phys_to_virt(dev_info.alloc_phys);
> >> -     kni->free_q = phys_to_virt(dev_info.free_phys);
> >> -
> >> -     kni->req_q = phys_to_virt(dev_info.req_phys);
> >> -     kni->resp_q = phys_to_virt(dev_info.resp_phys);
> >> -     kni->sync_va = dev_info.sync_va;
> >> -     kni->sync_kva = phys_to_virt(dev_info.sync_phys);
> >> +     if (dev_info.iova_mode) {
> >> +#if KERNEL_VERSION(4, 4, 0) > LINUX_VERSION_CODE
> >
> > We have "kni/compat.h" to put the version checks, please use
> > abstracted feature checks only in the code.
> > From experience this goes ugly quickly with the addition to distro
> > kernels and their specific versioning, so better to hide these all from the
> source code.
> >
> > And this version requirement needs to be documented in kni doc.
> >
> > ack
> >
> >> +             (void)pci;
> >> +             pr_err("Kernel version is not supported\n");
> >
> > Can you please include 'iova_mode' condition into the message log,
> > because this kernel version is supported if user wants to use via
> 'iova_mode == 0' condition.
> >
> > ack
> >
> >> +             return -EINVAL;
> >> +#else
> >> +             pci = pci_get_device(dev_info.vendor_id,
> >> +                                  dev_info.device_id, NULL);
> >> +             while (pci) {
> >> +                     if ((pci->bus->number == dev_info.bus) &&
> >> +                         (PCI_SLOT(pci->devfn) == dev_info.devid) &&
> >> +                         (PCI_FUNC(pci->devfn) ==
> >> +dev_info.function)) {
> >> +                             domain =
> >> +iommu_get_domain_for_dev(&pci->dev);
> >> +                             break;
> >> +                     }
> >> +                     pci = pci_get_device(dev_info.vendor_id,
> >> +                                          dev_info.device_id, pci);
> >> +             }
> >
> > What if 'pci' is NULL here?
> > In kni it is not required to provide a device at all.
> >
> > Ack, will add a NULL check.
> > other point is not clear to me, device info is absolutely required at
> > least for  IOVA=VA mode, since it requires to procure iommu domain
> details.
> 
> "device info is absolutely required" *only* for IOVA=VA mode, so user may
> skip to provide it.
> 
> > Any thoughts or ways to address this without device.?
> 
> Return error if 'iova_mode' requested but device info not?
> 
> But you didn't replied to passing 'iova_mode' from application, I would like
> hear what you are thinking about it..

One query regarding defining config for kni
Where this config comes, eal or kni sample app or KNI public API?

> 
> >
> > <...>
> >
> >> @@ -186,7 +202,10 @@ kni_fifo_trans_pa2va(struct kni_dev *kni,
> >>                        return;
> >>
> >>                for (i = 0; i < num_rx; i++) {
> >> -                     kva = pa2kva(kni->pa[i]);
> >> +                     if (likely(kni->iova_mode == 1))
> >> +                             kva = iova2kva(kni, kni->pa[i]);
> >> +                     else
> >> +                             kva = pa2kva(kni->pa[i]);
> >
> > To reduce the churn, what about updating the 'pa2kva()' and put the
> > "(kni->iova_mode == 1)" check there? Does it help? (not only
> > 'pa2kva()' but its friends also, and if it makes more sense agree to
> > rename the functions)
> >
> > No, in VA mode, kni->pa[i] points to iova address, pa2kva() of iova
> > address might crash, hence the if..else check is added.
> 
> I understand that part.
> What I am suggestion is something like this:
> 
> kva = common_function(kni, kni->pa[i]);
> 
> ---
> 
> common_function() {
> 	if (unlikely(kni->iova_mode == 1))
> 		return iova2kva(kni, kni->pa[i]);
> 	return pa2kva(kni->pa[i]);
> }
> 
> To hide the check in the function and make code more readable
> 
> >
> > And btw, why 'likely' case is "kni->iova_mode == 1"?
> >
> > no specific case other than branch predict, will remove this if it's
> > really harmful to PA mode.