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

[Jan Beulich <JBeulich@xxxxxxxx>]

On 26.07.2019 16:54, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <JBeulich@xxxxxxxx>
>> Sent: 26 July 2019 15:34
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) 
>> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> Subject: Re: [Xen-devel] per-domain passthrough/iommu options
>>
>> 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?
> 
> Yes... it's hard to express. What I want to say, of course, is that device 
> drivers can use GFNs. Can you think of any other form of words that might be 
> better?

Just omit "identity" in the HVM related sentence?

Jan
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