Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap

[Paolo Bonzini]

On 12/07/2017 03:07, Fam Zheng wrote:
> On Tue, 07/11 12:28, Paolo Bonzini wrote:
>> On 11/07/2017 12:05, Stefan Hajnoczi wrote:
>>> On Mon, Jul 10, 2017 at 05:08:56PM +0200, Paolo Bonzini wrote:
>>>> On 10/07/2017 17:07, Stefan Hajnoczi wrote:
>>>>> On Wed, Jul 05, 2017 at 09:36:32PM +0800, Fam Zheng wrote:
>>>>>> Allow block driver to map and unmap a buffer for later I/O, as a 
>>>>>> performance
>>>>>> hint.
>>>>> The name blk_dma_map() is confusing since other "dma" APIs like
>>>>> dma_addr_t and dma_blk_io() deal with guest physical addresses instead
>>>>> of host addresses.  They are about DMA to/from guest RAM.
>>>>>
>>>>> Have you considered hiding this cached mapping in block/nvme.c so that
>>>>> it isn't exposed?  block/nvme.c could keep the last buffer mapped and
>>>>> callers would get the performance benefit without a new blk_dma_map()
>>>>> API.
>>>>
>>>> One buffer is enough for qemu-img bench, but not for more complex cases
>>>> (e.g. fio).
>>>
>>> I don't see any other blk_dma_map() callers.
>>
>> Indeed, the fio plugin is not part of this series, but it also used
>> blk_dma_map.  Without it, performance is awful.
> 
> How many buffers does fio use, typically? If it's not too many, block/nvme.c 
> can
> cache the last N buffers. I'm with Stefan that hiding the mapping logic from
> block layer callers makes a nicer API, especially such that qemu-img is much
> easier to maintain good performance across subcommmands.

It depends on the queue depth.

I think the API addition is necessary, otherwise we wouldn't have added
the RAMBlockNotifier which is a layering violation that does the same
thing (create permanent HVA->IOVA mappings).  In fact, the
RAMBlockNotifier could be moved out of nvme.c and made to use
blk_dma_map/unmap, though I'm not proposing to do it now.

I don't think qemu-img convert and dd are impacted by IOMMU map/unmap as
heavily as bench, because they operate with queue depth 1.  But adding
map/unmap there would not be hard.

Paolo