Re: [Xen-devel] [PATCH v2 00/13] "Non-shared" IOMMU support on ARM

[Julien Grall <julien.grall@xxxxxxx>]

On 31/07/17 06:57, Tian, Kevin wrote:
> > From: Oleksandr Tyshchenko
> > Sent: Wednesday, July 26, 2017 1:27 AM
> >
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >
> > Hi, all.
> >
> > 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.
> Is "non-shared" IOMMU a standard terminology in ARM side? I quickly
> searched to find it mostly used in this thread...
>
> On the other hand, all IOMMUs support a basic DMA remapping
> mechanism with page table not shared with CPU. Then some IOMMUs
> may optional support Shared Virtual Memory (SVM) through page
> sharing with CPU. Then I'm not sure why need to highlight the
> "non-shared" manner in this thread, instead of just saying
> IPMMU-VMSA support...
This is not entirely true. You can share the page-table with the IOMMU 
as long as the page-table are the same. This may still involve some TLB 
flush on the IOMMU side, but you don't have allocate twice the 
page-table. Therefore you save memory.
This is actually the case of SMMUv{1,2}, we share the page table but 
still have to do the TLB flush and potential clean the page entry table.
Cheers,

--
Julien Grall

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