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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap
Date: Tue, 11 Jul 2017 11:05:35 +0100
User-agent: Mutt/1.8.0 (2017-02-23)

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.


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