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Re: [Xen-devel] [RFC PATCH v1 0/7] IPMMU-VMSA support on ARM

On 01/08/17 18:13, Oleksandr Tyshchenko wrote:
Hi, Julien

On Tue, Aug 1, 2017 at 3:27 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
On 26/07/17 16:09, Oleksandr Tyshchenko wrote:

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Hi, all.


Please CC maintainers and any relevant person on the cover letter. This is
quite useful to have in the inbox.
Yes. I CCed guys who, I think, are/were involved in IPMMU-VMSA
development from Linux side +
IOMMU maintainers (mostly ARM). Sorry, if I missed someone or mistakenly added.

The purpose of this patch series is to add IPMMU-VMSA support to Xen on
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 doesn't
use the same page-table format
as the CPU on ARM therefore I name it "Non-shared" IOMMU.
This all means that current patch series must be based on "Non-shared"
IOMMU support [1]
for the IPMMU-VMSA to be functional inside Xen.

The IPMMU-VMSA driver as well as the ARM LPAE allocator were directly
ported from BSP for Linux the vendor provides:

I think this is probably a good starting point to discuss about IOMMU
support in Xen. I skimmed through the patches and saw the words "rfc" and
"ported from BSP".
Well, at the time of porting IPMMU-VMSA driver, BSP [1] had more
complete support than mainline [2]
and seems to have at the moment.
For example, mainline driver still has single IPMMU context while BSP
driver can have up to 8 contexts,
the maximum uTLBs mainline driver can handle is 32, but for BSP driver
this value was increased to 48, etc.
But, I see attempts to get all required support in [3]. So, when this
support reaches upsteam, I hope,
it won't be a big problem to rebase on mainline driver if we decide to
align with it.

My main concern here is this driver haven't had a thorough review by the Linux community.

When we ported the SMMUv{1,2} driver we knew the Linux community was happy with it and hence adapting for Xen was not a big deal. There are only few limited changes in the code from Linux.

Looking at patch #2, #6, #7, the changes don't seem limited in the code from Linux + it is a driver from a BSP. The code really needs to be very close to make the port from Linux really worth it.

Stefano do you have any opinion?

At the moment for IOMMU we rely on the Linux community to do the review, but
this is not the case here as it is an RFC.  I can definitely help to check
if it comply for Xen,
yes, please.

but I don't have the competence to tell whether it is
valid for the hardware.

We may want to find a compromise to get it merged in Xen, but surely we
don't want to build it by default at least until we had feedback from the
community about the validity of the code here.

As I mentioned above, we are currently borrowing drivers from Linux and
adapting for Xen. Today we support SMMUv{1,2} (we need to resync it) and
there are plan to add IPMMU-VMSA (this series) and SMMUv3.
It would be really nice to have IPMMU-VMSA support in Xen. Without
this support the SCF [4] we are developing right now
and even the Passthrough feature won't be fully functional on R-Car
Gen3 based boards powered by Xen hypervisor.

As said in the previous e-mail. This would be nice enhancement for Xen, but we need to decide in which form it will be upstreamed.

I am aware that Linux IOMMU subsystem has growing quite a lot making more
tricky to get support in Xen. I wanted to get feedback how complex from you
and Sameer how complex it was and whether we should consider doing our own.

Yes, the IPMMU-VMSA Linux driver relies on some Linux functional
(IOMMU/DMA/io-pgtable frameworks) the Xen doesn't have (it is
expected). So, it took *some time*
to make Linux driver happy inside Xen). Moreover, this all resulted in
the fact that the driver looks complicated a bit).
A lot of different wrappers, #if 0, code style, etc.
On the other hand, I think, I will be able to fairly quickly align
with new BSP, etc.

But, I really don't know should we continue to follow this direction
or not, perhaps it will depend on
how complex the entity is and how much things we must pull together
with it to make it happy.


Julien Grall

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