Re: [Qemu-devel] [PATCH 0/4] introduce cpu_physical_memory_map_fast

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/06/2011 05:44 PM, Anthony Liguori wrote:
>>
>> Not just that. If you had a memory block at say 1 GB - 2 GB, and another
>> at 2 GB - 3 GB, a DMA operation that crosses the two could be
>> implemented with cpu_physical_memory_map_fast; you would simply build a
>> two-element iovec in two steps, something the current API does not allow.
>
> You cannot assume RAM blocks are contiguous.  This has nothing to do
> with PV or not PV but how the RAM API works today.

That's exactly why I said a *two-element* iovec.

> Virtio can handle all of this today because it uses
> cpu_physical_memory_rw for ring access and then calls map for SG
> elements.  SG elements are usually 4k so it's never really an issue to
> get a partial mapping.  We could be more robust about it but in
> practice, it's not a problem.

I know in practice it's not a problem, but I dislike not having an API 
that can deal with it even in theory.  For vmw_pvscsi it's like 5 lines 
of code to allow it.

Paolo