Re: [Xen-devel] [RFC PATCH 0/9] "Non-shared" IOMMU support on ARM

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

On Thu, Mar 16, 2017 at 5:31 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> On 03/15/2017 08:05 PM, Oleksandr Tyshchenko wrote:
>>
>> Hi, all.
>
>
> Hi Oleksandr,

Hi Julien

>
> Thank you for looking at the support of "non-shared" page table IOMMU
> support.
>
>>
>> The purpose of this patch series is to create a base for porting
>> any "Non-shared" IOMMUs to Xen on ARM. Saying "Non-shared" IOMMU I mean
>> the IOMMU that can't share the page table with the CPU.
>> Primarily, we are interested in IPMMU-VMSA and I hope that it will be the
>> first candidate.
>> It is VMSA-compatible IOMMU that integrated in the newest Renesas R-Car
>> Gen3 SoCs (ARM).
>> And this IOMMU can't share the page table with the CPU since it uses
>> stage-1 page table unlike the CPU that uses stage-2 therefore I name it
>> "Non-shared" IOMMU.
>>
>> I have already raised disscusion [2] about some generic problems I had
>> faced
>> during porting IPMMU-VMSA Linux driver to Xen. And on this basis I made
>> patches you can see it in this request. Only the first patch is not
>> related to
>> IOMMU. But, I decided to ship it with the current request since it is a
>> generic change
>> which we will need in a moment.
>>
>> The reason for this patch series to be RFC is that I still have some
>> doubts about generic code I touched.
>> I hope that I haven't broken anything for x86, but confirmation is needed.
>>
>> I didn't include IPMMU-VMSA driver in this request. Although, I am still
>> in progress, I want to say
>> that passthrough use-cases (actually what this all are firstly needed for)
>> work for me with some limitations.
>> I tested on Salvator-X board (R-Car H3) with the next devices that have
>> DMA IPs
>> connected to IPMMU uTLBs (using current master branch):
>> 1. AUDMA is assigned to dom0 (protected by IOMMU)
>> 2. SDHC0 is assigned to dom1 (passthrough to domain)
>> 3. SDHC3 is assigned to dom2 (passthrough to domain)
>
>
> I am looking forward to see the support in Xen :).

Yes, It would be really great.

>
>> During porting IPMMU-VMSA driver to Xen I was trying to be as close as
>> possible to Linux [1]. But,
>> it was a little bit difficult). It would be really nice to have some
>> feedback and get your feeling regarding this driver.
>> I am also interested in if I took the right direction or there are some
>> other ideas on doing the same.
>> So, is it a right direction to move on?
>
>
> I would be interested to know what was the difficulties to port Linux driver
> in Xen and how much you diverge from it.

From my point of view the difficulty is:
The IPMMU-VMSA Linux driver relies on some Linux functional
(IOMMU/DMA/io-pgtable frameworks)
the Xen doesn't have (it is expected). So, we have to dig into them
and try to understand how they works,
how they depend on each other, how they interact with the driver.
What we should get to Xen and how they will be integrated to it, or
does the Xen have alternative mechanisms we need to reuse,
or what functional won't be used in Xen at all and should be hidden
under #if 0.
So, we have to pull some of this stuff, make stubs/wrappers to make
driver happy inside Xen.
These all lead to the driver looks complicated a bit).

>
> My biggest concern is maintainability. It is fair to say that the Linux
> driver will be more exercised than the Xen driver. By keeping our own, we
> would potentially have hidden bug that may never be fixed.
>
> By the way, I am not saying we should stick to Linux. I am happy to consider
> both solutions :).

OK, the first possible solution (let's say direct port from Linux)
already exists in github,
and while I am in progress I can continue to follow this direction.
And I think I will be able to make the second variant (where I will
drop all Linux related stuff and
modify code not to rely on it) after I finish developing. Then we can
compare both variants and choose
one.
I would like to notice that I agree with both variants despite the
complexity of the first variant.

>
>>
>> You can find IPMMU-VMSA driver here.
>> repo: https://github.com/otyshchenko1/xen.git branch: ipmmu_ml
>> or
>> https://github.com/otyshchenko1/xen/commits/ipmmu_ml
>> It is located on top of "Unshared" IOMMU patch series and consist of 6
>> patches.
>
>
> I will have a look.

Looking forward to your and other guys comment.

>
> Cheers,
>
> --
> Julien Grall



-- 
Regards,

Oleksandr Tyshchenko

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