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

[Avi Kivity <avi@redhat.com>]

On 05/03/2011 07:49 PM, Paolo Bonzini wrote:
> Paravirtualized devices (and also some real devices) can assume they
> are going to access RAM.  For this reason, provide a fast-path
> function with the following properties:
>
> 1) it will never allocate a bounce buffer
>
> 2) it can be used for read-modify-write operations
>
> 3) unlike qemu_get_ram_ptr, it is safe because it recognizes "short" blocks
>
> Patches 3 and 4 use this function for virtio devices and the milkymist
> GPU.  The latter is only compile-tested.
>
> Another function checks if it is possible to split a contiguous physical
> address range into multiple subranges, all of which use the fast path.
> I will introduce later a use for this function.
>

Out of curiosity, what performance benefit do you see?

For relatively constant mappings (like the ring) we can cache the 
mapping in structure and invalidate it when the memory map changes 
(using, say, rcu).  That doesn't work for the actual buffers, or for 
indirect mappings.

-- 
error compiling committee.c: too many arguments to function