Re: [Qemu-devel] Re: [PATCH 1/5] Add target memory mapping API

[Ian Jackson]

Anthony Liguori writes ("Re: [Qemu-devel] Re: [PATCH 1/5] Add target memory 
mapping API"):
> Ian Jackson wrote:
> > I think my most recently sketched-out proposal (where the caller
> > passes a DMA sg list to the mapping request function) doesn't have
> > these deadlocks ?
> 
> But what do you do with a DMA request that cannot possibly be bounced 
> but that can be split?  In other words, a DMA request to 1G of MMIO 
> memory that is disk access.  We can absolutely split that into multiple 
> requests.

You impose a maximum size for any DMA request; that is, a maximum size
that callers of the DMA API are allowed to request.  Any larger
request is a bug.  Callers who might want to try to map bigger blocks
need to be able to split.

Alternatively you can have the API return an `amount mapped' on the
callback, but state that this number will always be at least
DMA_MAP_SG_GUARANTEE_MIN if at least that amount was requested.

Ian.