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Re: [Qemu-devel] [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_m

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap
Date: Tue, 11 Jul 2017 12:28:14 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0

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.


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