Re: [Qemu-devel] [PATCH 00/10] SCSI scatter/gather support

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Aug 04, 2011 at 07:14:38PM +0200, Paolo Bonzini wrote:
> Hi,
> 
> this is the version of SCSI scatter/gather based on the existing
> DMA helpers infrastructure.
> 
> The infrastructure required a little update because I need to
> know the residual amount of data upon short transfers.  To this
> end, my choice was to make QEMUSGList mutable and track the
> current position in there.  Any other ideas are welcome, the
> reason for this choice is explained in patch 2.
> 
> The patches are quite self-contained, but they depend on the
> changes I posted yesterday.
> 
> Patch 11 is the sample vmw_pvscsi device model that I used to
> test the code.

This is a good opportunity to rename is_write in dma-helpers because it
is confusing.

The problem is that bdrv_*() is_write indicates whether the I/O request
is a read or write.  But in cpu_physical_memory_map() is_write indicates
whether we are writing to target memory.

These two is_write use cases actually have opposite meanings, therefore
the confusing code in dma-helpers.c today:
mem = cpu_physical_memory_map(cur_addr, &cur_len, !dbs->is_write);
                                                  ^^^^^^^^^^^^^^

Please use a DMA direction instead of is_write:

DMA-to-device means target->device memory transfer
DMA-from-device means device->target memory transfer

This patch series is a good place to do the rename because it adds more
instances of !dbs->is_write.

Stefan