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

[Julien Grall <julien.grall@xxxxxxx>]

On 03/15/2017 08:05 PM, Oleksandr Tyshchenko wrote:
> Hi, all.
Hi Oleksandr,

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 :).

> 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.
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 :).
> 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.

Cheers,

--
Julien Grall

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