Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

[Eric Blake]

On 08/11/2017 03:09 AM, Fam Zheng wrote:
> People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> header. While this is natural because raw doesn't need to write any
> magic bytes during creation, hdev_create is free to clear out the first
> sector to make sure the stale qcow2 header doesn't cause such confusion.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> 
> ---
> 
> v2: Use stack allocated buffer. [Eric]
>     Fix return value.
>     (Keep qemu_write_full instead of switching to qemu_pwritev because
>     the former handles short writes.)
>     Fix typo "qemu-img". [Changlong]
> ---
>  block/file-posix.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f4de022ae0..a63bbf2b90 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts 
> *opts,
>          ret = -ENOSPC;
>      }
>  
> +    if (total_size) {
> +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> +        if (lseek(fd, 0, SEEK_SET) == -1) {
> +            ret = -errno;
> +        } else {
> +            ret = qemu_write_full(fd, buf, zero_size);
> +            ret = ret == zero_size ? 0 : -errno;
> +        }
> +    }

Question: are we ever constrained by O_DIRECT where writing only 512
bytes would be too small for a block device that mandates 4k alignment?
If so, then we need MAX(minimum write size, MIN(BDRV_SECTOR_SIZE,
total_size)) - it would also mean we can't stack-allocate any more, but
that we have to do an aligned buffer allocation (where g_malloc is not
necessarily suitably aligned).

If O_DIRECT is not a problem, then this is okay:

Reviewed-by: Eric Blake <address@hidden>

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

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