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

[Paolo Bonzini]

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).

Paolo

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