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Re: [RFC PATCH v2 10/26] qcow2: Update get/set_l2_entry() and add get/se

From: Alberto Garcia
Subject: Re: [RFC PATCH v2 10/26] qcow2: Update get/set_l2_entry() and add get/set_l2_bitmap()
Date: Thu, 14 Nov 2019 14:57:54 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Wed 30 Oct 2019 05:55:04 PM CET, Max Reitz wrote:
>> This patch also adds the get/set_l2_bitmap() functions that are used
>> to access the bitmaps. For convenience, these functions are no-ops
>> when used in traditional qcow2 images.
> Granted, I haven’t seen the following patches yet, but if these
> functions are indeed called for images that don’t have subclusters,
> shouldn’t they return 0x0*0f*f then? (i.e. everything allocated)
> If they aren’t, they should probably just abort().  Well,
> set_l2_bitmap() should probably always abort() if there aren’t any
> subclusters.

Yeah, set_l2_bitmap() should abort (I had this changed already).

About get_l2_bitmap() ... I decided not to abort for convenience, for
cases like this one:

    uint64_t l2_entry = get_l2_entry(s, l2_slice, l2_index);
    uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
    type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc);

Here the value of l2_bitmap is going to be ignored anyway so it doesn't
matter what we return, but perhaps for consistency we should return
QCOW_OFLAG_SUB_ALLOC(0), which means that the first (and only, in this
case) subcluster is allocated.


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