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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap
Date: Mon, 10 Jul 2017 16:07:07 +0100
User-agent: Mutt/1.8.0 (2017-02-23)

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()

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