Re: [Xen-devel] Question about PLATFORM_QUIRK_DOM0_MAPPING_11

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-09-04 at 20:30 +0800, Chen Baozi wrote:
> On Sep 3, 2013, at 8:49 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 
> > On Tue, 2013-09-03 at 20:41 +0800, Chen Baozi wrote:
> >> When porting Xen on OMAP5, we need to add .quirks handler which
> >> returns the PLATFORM_QUIRK_DOM0_MAPPING_11. I have read the related
> >> set_memory_reg() in domain_build.c. And My question is in which case
> >> we don't need to return PLATFORM_QUIRK_DOM0_MAPPING_11 in .quirks and
> >> how Xen handles that case to populate right P2M to dom0?
> > 
> > DOM0_MAPPING_11 is a workaround for lack of an iommu, either in hardware
> > (e.g. ARM SMMU) or in software (e.g. Stefano's swiotlb patches). In the
> > short term it seems like Stefano's patches are going to be ready before
> > we have an SMMU driver in the hypervisor.
> > 
> > In any case DOM0_MAPPING_11 will hopefully go away before too long.
> 
> Thanks a lot. And one more question :P
> 
> The reason that we cannot boot successfully without DOM0_MAPPING_11 on
> OMAP5 is because in that case the physical address dom0 writes to DMA
> controller is intermediate physical address rather than physical address.
> If we did the 1:1 mapping, the IPA of dom0 is then equal to the PA, in
> which case the DMA can work properly in dom0. Is this the right explanation
> why we need PLATFORM_QUIRK_DOM0_MAPPING_11 without either SMMU or swiotlb?

Correct. 

BTW, ballooning dom0 tends to destroy the 1:1 map, so it's by no means a
perfect bandaid.

Ian.


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