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Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_d
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap |
Date: |
Wed, 12 Jul 2017 16:03:57 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 |
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
- Re: [Qemu-block] [PATCH v3 2/6] block: Add VFIO based NVMe driver, (continued)
[Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Fam Zheng, 2017/07/05
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Stefan Hajnoczi, 2017/07/10
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Stefan Hajnoczi, 2017/07/10
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Paolo Bonzini, 2017/07/10
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Stefan Hajnoczi, 2017/07/11
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Paolo Bonzini, 2017/07/11
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Fam Zheng, 2017/07/11
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap,
Paolo Bonzini <=
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Stefan Hajnoczi, 2017/07/14
- Re: [Qemu-block] [PATCH v3 3/6] block: Introduce bdrv_dma_map and bdrv_dma_unmap, Paolo Bonzini, 2017/07/14
[Qemu-block] [PATCH v3 4/6] block/nvme: Implement .bdrv_dma_map and .bdrv_dma_unmap, Fam Zheng, 2017/07/05
[Qemu-block] [PATCH v3 5/6] qemu-img: Map bench buffer, Fam Zheng, 2017/07/05
[Qemu-block] [PATCH v3 6/6] block: Move NVMe spec definitions to a separate header, Fam Zheng, 2017/07/05