Re: [dpdk-dev] [PATCH 2/2] eal: fix IOVA mode selection as VA for pci drivers

[Thomas Monjalon <thomas@monjalon.net>]

15/07/2019 17:35, Jerin Jacob Kollanukkaran:
> From: Thomas Monjalon <thomas@monjalon.net>
> > 15/07/2019 16:26, Jerin Jacob Kollanukkaran:
> > > > > Is there any specific reason why we always prefer PA if physical
> > > > > addresses are available? Since we're already assuming that all
> > > > > devices support PA and VA anyway, what's the harm in enabling VA by
> > default?
> > > >
> > > > If PA is available, it means we are running as root.
> > > > We can assume that using root is a choice, probably related to a
> > > > preference for PA.
> > >
> > > # Even if we are running as root, Why to choose PA in case of DC?
> > > ie. Following logic is not need
> > >                 if (iova_mode == RTE_IOVA_DC) {
> > >                         iova_mode = phys_addrs ? RTE_IOVA_PA : RTE_IOVA_VA;
> > >                         RTE_LOG(DEBUG, EAL,
> > >                                 "Buses did not request a specific IOVA mode, using '%s'
> > based on physical addresses availability.\n",
> > >                                 phys_addrs ? "PA" : "VA");
> > >                 }
> > 
> > Why running as root if using VA anyway?
> > We can assume the user knows what he is doing, so it is a user choice.
> > We want to allow the user choosing, right?
> 
> The user can override iova=pa/va as eal argument if user needs to run a specific mode.
> Running as root for various other reason(just be lazy) etc. it is not or it should not
> be connected to set the mode as PA.

Good point.
I tend to prefer avoiding the use of EAL arguments because they may
be unavailable, depending on the application.

> > > # When DPDK running on guest, Anyway it can not access the real PA, It will
> > be IPA.
> > 
> > What is IPA? Isn't it a beer?
> 
> There may a beer with that name. In this context, it is "Intermediate physical address"
> 
> > > So I don't understand logic behind choose PA when DC.
> > > To me, it make sense to choose PA when DC.
> > 
> > You probably mean "choose VA".
> 
> Yup.
> 
> > > # To align with RTE_PCI_DRV_NEED_MAPPING flag and reflect it "need"
> > > rather than support, I think, flag can be changed to
> > > RTE_PCI_DRV_NEED_IOVA_AS_VA
> > 
> > I think the most important is to have a good documentation of this flag (it
> > was not done properly when Cavium introduced it initially).
> > If you want to rename the flag, you can do it in a separate patch.
> > If renaming, I really would like to get an answer to an old question:
> > Why IO adress is called IOVA? The name "IOVA_AS_VA" looks strange.
> 
> IOVA = IO virtual address
> Since IOVA can be PA or VA, the name IOVA_AS_VA as chosen 

We could also call it "bus address" or "device address".
I think the word "IOVA" was enforced by Linux.
Anyway, my real issue when using "virtual" is that we don't really
know what we are talking about: is it an IOMMU translated address
on the device side or an MMU translated address on the application side?

I think we should better explain things.
One diagram which can help:
https://en.wikipedia.org/wiki/Input%E2%80%93output_memory_management_unit#/media/File:MMU_and_IOMMU.svg

> > For reference, one description of addressing:
> > https://lists.linuxfoundation.org/pipermail/iommu/2018-May/027686.html
> > 
> > About the naming, do you remember how I insisted to have a correct naming
> > of all related stuff in DPDK? It was hard to get it accepted, the discussion was
> > not nice and I stopped insisting to get all details fine because I just got bored.
> > It was a really bad experience.
> 
> I agree.
> To me that bad experience was due to mostly not having enough technical comments
> On the proposal. Though I am not the author/owner of it.
> 
> > You can ask why I remind this now? Because we must take care of all details,
> > make sure our messages are well understood, and be cooperative.
> 
> No disagreement.
> If we see the history the meaning got changed/updated in this commit
> By adding intel drivers to it. I would nt say  it is big ideal, It just C code,
> It can be changed based on the need. I think, what really import is,
> maintain the the feature and commitment towards fixing any issue.
> 
> commit f37dfab21c988d2d0ecb3c82be4ba9738c7e51c7
> Author: Jianfeng Tan <jianfeng.tan@intel.com>
> Date:   Wed Oct 11 10:33:48 2017 +0000
> 
>     drivers/net: enable IOVA mode for Intel PMDs
> 
>     If we want to enable IOVA mode, introduced by
>     commit 93878cf0255e ("eal: introduce helper API for IOVA mode"),
>     we need PMDs (for PCI devices) to expose this flag.
> 
>     Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
>     Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
>     Reviewed-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>

The doxygen meaning did not change from day one:
	/** Device driver supports IOVA as VA */
But the commit log meaning was:
	"Flag used when driver needs to operate in iova=va mode."
And the Intel commit log had a different understanding:
	"If we want to enable IOVA mode, [..] we need PMDs [..] to expose this flag."

Anyway we agree on the new meaning to be the original one the author
had in mind (i.e. "driver needs").

> > > Other than above points,
> > > Reviewed this patch and tested on octeontx2, It looks good to me.