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

[Anthony Liguori]

Ian Jackson wrote:
> 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.
>   

For that to be useful for network traffic, it has to be at least 64k.  
If isn't at least 64k, then the NIC code must deal with two cases, one 
where it was able to map and one where it wasn't able to map.

In the general case, we can't know ahead of time what the largest MIN is 
to make all devices happy.

Regards,

Anthony Liguori

> Ian.
>
>
>