Re: [Xen-devel] [PATCH 5/25] Xen/doc: Add Xen virtual IOMMU doc

[Lan Tianyu <tianyu.lan@xxxxxxxxx>]

On 2017年07月08日 00:16, Julien Grall wrote:
> Hi,
> 
> On 06/07/17 07:20, Lan Tianyu wrote:
>> On 2017年07月05日 21:25, Julien Grall wrote:
>>> Furthermore, on ARM we would be able to create the vIOMMU but it would
>>> be unusable. Indeed, IOMMU are only used to protect devices. But you
>>> don't see any way to say "This device is protected by the IOMMU". Did I
>>> miss anything?
>>
>> The "device protection" you mentioned is DMA protection, right?. It's
>> one of IOMMU capabilities. IOMMU also provides interrupt remapping and
>> SVM(Shared virtual memory). I see ARM side also is pushing SVM feature
>> in KVM maillist for native support. Finally, it needs to support SVM in
>> VM and so virtual IOMMU is necessary regardless of full-virtualized or
>> PV IOMMU
>>
>> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-March/491614.html
>>
> 
> I don't think SVM is strictly necessary to do DMA protection in the
> guest.

SVM and DMA protection is different features of IOMMU. SVM is to share
same page table(VA->PA) between CPU and GPU or other device in order to
removing overhead to maintain two page table between cpu and device
side. Actually this is also a device feature and more devices will
support SVM besides GPU.

> Not all IOMMUs on ARM are able to use this feature but you may
> still want to allow the guest using the IOMMU. Did I miss anything?

If physical IOMMU doesn't support SVM, vIOMMU device model should not
return SVM capability to tool stack when receive "query capabilities"
cmd. There should be capabilities field in vIOMMU register or ACPI table
for vIOMMU(Not sure ARM side) and SVM capability bit won't be set.
So guest finally won't enable SVM feature.

-- 
Best regards
Tianyu Lan

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