Re: [Qemu-devel] [PATCH 05/10] dma-helpers: add dma_buf_read and dma_buf_write

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/11/2011 09:58 AM, Stefan Hajnoczi wrote:
> On Thu, Aug 04, 2011 at 07:14:43PM +0200, Paolo Bonzini wrote:
>> These helpers do a full transfer from an in-memory buffer to
>> target memory, with full support for MMIO areas.  It will be used to store
>> the reply of an emulated command into a QEMUSGList provided by the
>> adapter.
>>
>> Signed-off-by: Paolo Bonzini<pbonzini@redhat.com>
>> ---
>>   cutils.c      |    8 +++---
>>   dma-helpers.c |   63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>   dma.h         |    5 ++++
>>   3 files changed, 72 insertions(+), 4 deletions(-)
>
> I don't understand this patch.  If we have a memory buffer that needs to
> be transferred to target memory, then there is no need for bounce
> buffers or cpu_physical_memory_map().
>
> Can we use cpu_physical_memory_rw() on each sglist element instead?  No
> -EAGAIN necessary because the memory buffer already acts as the local
> bounce buffer.

Doh, you're obviously right.  I don't know what I was thinking. :)

What do you think about passing the residual bytes for short transfers? 
  Should I look into updating BlockDriverCompletionFunc, or is the 
approach of patch 2 okay?  If I have an excuse to learn more about 
Coccinelle, that can be fun. :)

Paolo