Re: [Xen-devel] Question about porting IPMMU-VMSA Linux driver to XEN

[Julien Grall <julien.grall@xxxxxxx>]

On 08/12/16 17:06, Oleksandr Tyshchenko wrote:
> Hi Julien,
Hi Oleksandr,

As discussed on IRC, I CCed xen-devel and Stefano.

> We would like to hear your opinion about the proper way of porting
> kernel driver to XEN.
> There is a Linux iommu driver "IPMMU VMSA" for supporting
> VMSA-compatible IPMMUs that are integrated in the newest Renesas SoCs.
> Mainline:
> http://lxr.free-electrons.com/source/drivers/iommu/ipmmu-vmsa.c
> But we would prefer to rely on code that hasn't reach upstream yet but
> shipped with BSP for this platform and seems to be more complete:
> https://git.kernel.org/cgit/linux/kernel/git/horms/renesas-bsp.git/tree/drivers/iommu/ipmmu-vmsa.c?h=v4.6/rcar-3.3.x
>
> For passthrough and future Remote processor (coproc) use-cases to work
> on R-Car Gen3 based platforms we need this driver to be ported to XEN.
> But I am in doubt how to do this in a right way.
>
> So, from my point of view there are two possible ways:
> 1. Try to keep this driver as close as possible from Linux like you
> did for arm-smmu driver. Even keeping the Linux style. I understand
> the main goal despite the overhead and so on.
> 2. Another way is to try to be as similar as possible from arm-smmu
> driver in XEN. In such case many common for both drivers things might
> be moved to the common part.
I don't think you would be able to share a lot of code between arm-smmu 
and ipmmu-vmsa. The former is using stage-2 page table whilst ipmmu-vmsa 
is using stage-1 page table. So the page table would have to be unshared 
in your case. This is something we don't yet support on Xen ARM.
Furthermore, I still want to keep arm-smmu as close as possible from 
Linux mainline. When I first implemented the driver I chose to not stick 
on Linux, but we decided to re-sync few months later because it was too 
hard to maintain. One of the main advantage of keeping close to Linux is 
we can backport bug more easily.
I would prefer to see ipmmu-vmsa as close as Linux (BSP or mainline), 
but this is not a strong requirement. Do you know why the changes you 
are looking for are not yet upstreamed? What are the differences?
Cheers,

--
Julien Grall

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