Re: [Qemu-block] [PATCH v6 3/9] block: Add VFIO based NVMe driver

[Eric Blake]

On 01/16/2018 12:08 AM, Fam Zheng wrote:
> This is a new protocol driver that exclusively opens a host NVMe
> controller through VFIO. It achieves better latency than linux-aio by
> completely bypassing host kernel vfs/block layer.
>
>      $rw-$bs-$iodepth  linux-aio     nvme://
>      ----------------------------------------
>      randread-4k-1     10.5k         21.6k
>      randread-512k-1   745           1591
>      randwrite-4k-1    30.7k         37.0k
>      randwrite-512k-1  1945          1980
>
>      (unit: IOPS)
>
> The driver also integrates with the polling mechanism of iothread.
>
> This patch is co-authored by Paolo and me.
>
> Signed-off-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> Message-Id: <address@hidden>
> ---

Sorry for not noticing sooner, but

> +static int64_t coroutine_fn nvme_co_get_block_status(BlockDriverState *bs,
> +                                                     int64_t sector_num,
> +                                                     int nb_sectors, int *pnum,
> +                                                     BlockDriverState **file)
> +{
> +    *pnum = nb_sectors;
> +    *file = bs;
> +
> +    return BDRV_BLOCK_ALLOCATED | BDRV_BLOCK_OFFSET_VALID |
> +           (sector_num << BDRV_SECTOR_BITS);

This is wrong.  Drivers should only ever return BDRV_BLOCK_DATA (which 
io.c then _adds_ BDRV_BLOCK_ALLOCATED to, as needed).  I'll fix it up as 
part of my byte-based block status series (v8 coming up soon).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org