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Re: [Qemu-devel] [PATCH v7 11/25] qcow2: refcount_order parameter for qc
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v7 11/25] qcow2: refcount_order parameter for qcow2_create2 |
Date: |
Wed, 18 Feb 2015 19:51:05 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
On 02/18/2015 03:40 PM, Max Reitz wrote:
> Add a refcount_order parameter to qcow2_create2(), use that value for
> the image header and for calculating the size required for
> preallocation.
>
> For now, always pass 4.
>
> This addition requires changes to the calculation of the file size for
> the "full" and "falloc" preallocation modes. That in turn is a nice
> opportunity to add a comment about that calculation not necessarily
> being exact (and that being intentional).
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> block/qcow2.c | 47 ++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 36 insertions(+), 11 deletions(-)
>
> @@ -2010,6 +2022,8 @@ static int qcow2_create(const char *filename, QemuOpts
> *opts, Error **errp)
> size_t cluster_size = DEFAULT_CLUSTER_SIZE;
> PreallocMode prealloc;
> int version = 3;
> + uint64_t refcount_bits = 16;
> + int refcount_order;
> Error *local_err = NULL;
> int ret;
>
> @@ -2064,8 +2078,19 @@ static int qcow2_create(const char *filename, QemuOpts
> *opts, Error **errp)
> goto finish;
> }
>
> + if (version < 3 && refcount_bits != 16) {
> + error_setg(errp, "Different refcount widths than 16 bits require "
> + "compatibility level 1.1 or above (use compat=1.1 or "
> + "greater)");
> + ret = -EINVAL;
> + goto finish;
> + }
> +
> + refcount_order = ffs(refcount_bits) - 1;
ffs() doesn't work on uint64_t (it gives the wrong answer for
0x100000000, for example); you want to use ffsll(). But ffsll() isn't
portable. But we have include/qemu/host-utils.h that gives us ctz64()
which is what we want (where ctz==ffs-1 other than for the special case
of 0). So this should be:
refcount_order = ctz64(refcount_bits);
With that change,
Reviewed-by: Eric Blake <address@hidden>
(Hmm, that means that at least qcow2.c:qcow2_create2() has a bug for
calling ffs(size_t), and we probably ought to eradicate other uses of
ffs from the tree - but as a separate followup).
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- [Qemu-devel] [PATCH v7 00/25] qcow2: Support refcount orders != 4, Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 12/25] qcow2: Use symbolic macros in qcow2_amend_options, Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 13/25] iotests: Prepare for refcount_bits option, Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 15/25] iotests: Add test for different refcount widths, Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 16/25] progress: Allow regressing progress, Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 18/25] qcow2: Use error_report() in qcow2_amend_options(), Max Reitz, 2015/02/18
- [Qemu-devel] [PATCH v7 14/25] qcow2: Allow creation with refcount order != 4, Max Reitz, 2015/02/18