RE: [Xen-devel] [PATCH 0/5] VT-d support for PV guests

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Ian Pratt
>Sent: 2008年5月20日 23:34
>> > It would be good if you could provide a bit more detail on when the
>> > patch populates IOMMU entries, and how it keeps them in sync. For
>> > example, does the IOMMU map all the guest's memory, or just that
>> > which will soon be the subject of a DMA? How synchronous is the
>> > patch in removing mappings, e.g. due to page type changes 
>(pagetable
>> > pages, balloon driver) or due to unmapping grants?
>> 
>> All writable memory is initially mapped in the IOMMU.  Page type
>> changes are also reflected there.  In general all maps and 
>unmaps to a
>> domain are synced with the IOMMU.  According to the feedback I got I
>> apparently missed some places, though.  Will look into this and fix
>> it.
>
>Is "demotion" of access handled synchronously, or do you have some
>tricks to mitigate the synchronization?

All changes need be handled synchronously, as DMA request is not
restartable with VT-d fault as async event notification. Hardware bits
are designed in such way that all expected permission controls have
to exist before device actually issues access request. 

>
>> It's clear that performance will pretty much suck if you do frequent
>> updates in grant tables, but the whole idea of having passthrough
>> access for NICs is to avoid this netfront/netback data plane scheme
>> altogether.  This leaves you with grant table updates for 
>block device
>> access.  I don't know what the expected update frequency is for that
>> one.
>
>I don't entirely buy this -- I think we need to make grant map/unmaps
>fast too. We've discussed schemes to make this more efficient by doing
>the IOMMU operations at grant map time (where they can be easily
>batched) rather than at dma_map time. We've talked about using a

Agree.

>kmap-style area of physical address space to cycle the mappings through
>to avoid having to do so many synchronous invalidates (at the 
>expense of
>allowing a driver domain to be able to DMA to a page for a 
>little longer
>than it strictly ought to).

Could you elaborate a bit how kmap-style area helps here? The key
point is whether frequency of p2m mapping can be reduced...

Thanks,
Kevin 

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