[Qemu-devel] Re: [PATCH 4/8] ahci: use qiov instead of dma helpers

[Alexander Graf <agraf@suse.de>]

On 18.01.2011, at 13:35, Kevin Wolf wrote:

> Am 20.12.2010 22:13, schrieb Alexander Graf:
>> The DMA helpers incur additional overhead on data transfers. I'm not
>> sure we need the additional complexity provided by them. So let's just
>> use qiovs directly when running in the fast path (ncq).
>> 
>> Signed-off-by: Alexander Graf <agraf@suse.de>
>> ---
>> hw/ide/ahci.c |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>> hw/ide/ahci.h |    3 ++
>> 2 files changed, 95 insertions(+), 8 deletions(-)
> 
> I don't feel comfortable with this one, and I think a while ago we
> discussed on IRC why the DMA helpers even exist. If AHCI doesn't need
> them, probably nobody needed them.
> 
> However, I'm inclined to think that AHCI actually _does_ need them in
> corner cases, even though it might not break in all the common cases
> that you have tested. Can you explain why only AHCI doesn't need it or
> is it just "didn't break for me in practice"?
> 

It's the latter.

> Where does the overhead in the DMA helpers come from? Can we optimize
> this code instead of making the device emulation less correct?

Well, dma helpers involve another malloc which is probably the biggest hog. I frankly don't see the point in making it correct for the fast path though. I'd rather like to have a fast block emulation that works with all OSs than an accurate one that emulates something nobody cares about.

Virtio for example doesn't use dma helpers either - they just claim it's not defined in the spec. So if virtio-blk gets away with it, it means that all OSs should never make use of the additional complexity.


Alex