Re: [Xen-devel] Xen ARM smmu questions

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Hi Jinch,

Replies inline below.


On Tue, 12 Mar 2019, Jinch wrote:
> Thank you for your reply,
> I’m just researching the implementation of the SMMU driver in Xen and dom0 
> kernel.
> If my board has SMMU of arm,mmu-500, do you mean that the Xen use the stage 2 
> of it and the dom0 kernel use stage1?

Yes, that is the idea, however, it is not straightforward to implement
from what I have been told. For your information, today, only stage2 in
Xen works -- stage1 cannot be used by dom0 (or anybody else) and would
be left unused.


> And if I want to passthrough some device that use SMMU to domu, how the SMMU 
> working in Xen and the domu kernel?

As I mentioned, today Xen makes use of the SMMU, and nobody else in the
system can access it, not dom0, not any domUs. I would love to have a
way to export the SMMU to dom0 and/or a domU. To do that, the easiest
way would be to introduce support for a paravirtualized IOMMU as Julien
mentioned: Xen would expose a "fake" SMMU to guests, and based on its
settings, it would program the real SMMU (it could even use stage1 to do
it). The "fake" SMMU is not implemented yet.


> > 在 2019年3月12日，下午7:42，Julien Grall <julien.grall@xxxxxxx> 写道：
> > 
> > 
> > On 3/12/19 8:58 AM, jinchen wrote:
> >> Hello xen-devel,
> > 
> > Hello,
> > 
> >> I'm reading the code of xen arm smmu in drivers/passthrough/arm, and I 
> >> have some questions that confused me.
> >> I think if the board use SMMU, xen will take charge of it before dom0 
> >> boot, and will not pass the node of SMMU to dom0 in device tree,
> >> so my question is how dom0 use SMMU? or can dom0 use SMMU? What dom0 use 
> >> if it wants to use IOMMU?
> >> What about domU? The SMMU could be passed to domU?
> > 
> > Some SMMU are able to provide Stage 1 (Guest VA to Guest PA) and Stage 2 
> > (Guest PA to Host PA) support. In the current configuration, Xen will use 
> > the Stage 2 part to isolate device passthrough. Stage 1 is not supported.
> > 
> > If you wanted a guest to use the SMMU, you would either need to provide 
> > Stage 1 support (requires HW support) or a PV IOMMU.
> > 
> > Alternatively, you might be able to give the SMMU to Dom0. I haven't 
> > explored it so far. Do you have a use case for using the IOMMU in guest?
> > 
> > Cheers,
> > 
> > -- 
> > Julien Grall
> > 
> 
> 
>

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