Re: [Xen-devel] per-domain passthrough/iommu options

[Jan Beulich <JBeulich@xxxxxxxx>]

On 26.07.2019 16:26, Paul Durrant wrote:
>> From: Jan Beulich <JBeulich@xxxxxxxx>
>> Sent: 26 July 2019 15:02
>>
>> On 26.07.2019 15:39, Paul Durrant wrote:
>>> ? I don't think 'private-pt' or 'separate-pt' really capture the fact that 
>>> the page tables match the
>> P2M. They could just as easily be taken to mean that they are populated 
>> using some other policy.
>>
>> But haven't we recently seen that this fully lock-step population
>> of page tables isn't always correct (or at least desirable)? I
>> vaguely recall other comments to that effect too, from long ago.
>> I'd specifically want to avoid encoding into the interface here
>> that the two are exact mirrors of one another, now and forever.
> 
> How do you think we should express it. I agree that it's a bit awkward 
> because of the difference between HVM and PV domains, but all we can do there 
> really is document it I think, so perhaps the manpage could have something 
> like:

Sounds reasonable - it at least avoids making the behavior too
much spelled out with regard to the similarity of mappings between
IOMMU page tables and P2M. There's one issue though:

> 'off'
> 
> IOMMU mappings are disabled for the domain and so hardware may not be passed 
> through.
> 
> 'sync-pt'
> 
> For a PV domain, all writable pages assigned to the domain are identity 
> mapped by MFN in the IOMMU page tables. Thus a device driver running in the 
> domain may program passthrough hardware for DMA using MFN values (i.e. 
> host/machine frame numbers) looked up in its P2M.
> For an HVM domain, all non-foreign RAM pages present in the P2M will be 
> identity mapped by GFN

Why "identity mapped" here? It's a GFN -> MFN mappingm, isn't it?

Jan

> in the IOMMU page tables. Thus a device driver running in the domain may 
> program passthrough hardware using GFN values (i.e. guest physical frame 
> numbers) without any further translation.
> 
> 'share-pt'
> 
> This is unavailable for a PV domain. For an HVM domain, this option means 
> that the IOMMU will be programmed to directly reference the P2M as its page 
> tables. This availability of this option is hardware specific and thus, if it 
> is specified for a domain running on hardware that does not allow it, 
> 'sync-pt' will be used instead.
> 
> ?
> 
>    Paul
> 
>>
>> Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel