Re: [Xen-devel] [RFC PATCH 9/9] xen: Add use_iommu flag to createdomain domctl

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

Hi, Wei

On Tue, Apr 25, 2017 at 6:23 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> On Wed, Apr 19, 2017 at 07:26:44PM +0100, Julien Grall wrote:
>> Hi Oleksandr,
>>
>> Please CC the appropriate maintainers for all the components you modify.
>>
>> On 15/03/17 20:05, Oleksandr Tyshchenko wrote:
>> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>> >
>> > This flag is intended to let Xen know that the guest has devices
>> > which will most likely be used for passthrough.
>> > The primary aim of this knowledge is to help the IOMMUs that don't
>> > share page tables with the CPU be ready before P2M code starts
>> > updating IOMMU mapping.
>> > So, if this flag is set the unshared IOMMUs will populate their
>> > page tables at the domain creation time and thereby will be able
>> > to handle IOMMU mapping updates from *the very beginning*.
>> >
>
> If it is not able to do so since the very beginning, what will happen?
IOMMU page faults will occur if passthroughed/protected devices try to
do DMA since
the corresponding memory mapping won't be present in IOMMU page table.
If we don't populate IOMMU page table (with setting need_iommu flag)
during domain creation time we will lose memory mapping (at least
domain RAM).
Unfortunately, we can't restore this memory mapping on ARM unlike x86.
I would like to say that it is true for non-shared IOMMUs only. For
shared IOMMU (SMMU) we don't have such problem.

>
> Let me explain where I'm coming from:
>
> 1. if not populating the iommu page table causes Xen to malfunction
>    (crash?), it is a bug. It should be fixed without involvement
>    from toolstack.
> 2. if this is an optimasation, can't we not always populate iommu pt
>    hence no new flag is needed?
>
> Overall I'm not too convinced a new flag is needed.
We don't want to always populate IOMMU page table since it might be
just wasting CPU and memory resources if no devices are assigned to
domain. That's why we need this hint.

>
> Wei.



-- 
Regards,

Oleksandr Tyshchenko

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