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

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/06/2011 02:56 PM, Anthony Liguori wrote:
>
> Oh, the patch series basically died for me when I saw:
>
> Avi> What performance benefit does this bring?
>
> Paolo> Zero

:)

> Especially given Avi's efforts to introduce a new RAM API, I don't want
> yet another special case to handle.

This is not a special case, the existing functions are all mapped onto 
the new cpu_physical_memory_map_internal.  I don't think this is in any 
way related to Avi's RAM API which is (mostly) for MMIO.

> You're just trying to avoid having to handle map failures, right?

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.

The patch does not change virtio to do the split, but it is possible to 
do that.  The reason I'm not doing the virtio change, is that I know mst 
has pending changes to virtio and I'd rather avoid the conflicts for 
now.  However, for vmw_pvscsi I'm going to handle it using the new 
functions.

Paolo