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Re: [Qemu-block] [PATCH v3 17/39] qcow2: Update l2_allocate() to support

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH v3 17/39] qcow2: Update l2_allocate() to support L2 slices
Date: Thu, 01 Feb 2018 14:13:01 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Wed 31 Jan 2018 09:07:48 PM CET, Max Reitz wrote:
> On 2018-01-26 15:59, Alberto Garcia wrote:
>> This patch updates l2_allocate() to support the qcow2 cache returning
>> L2 slices instead of full L2 tables.
>> The old code simply gets an L2 table from the cache and initializes it
>> with zeroes or with the contents of an existing table. With a cache
>> that returns slices instead of tables the idea remains the same, but
>> the code must now iterate over all the slices that are contained in an
>> L2 table.
>> Since now we're operating with slices the function can no longer
>> return the newly-allocated table, so it's up to the caller to retrieve
>> the appropriate L2 slice after calling l2_allocate() (note that with
>> this patch the caller is still loading full L2 tables, but we'll deal
>> with that in a separate patch).
>> Signed-off-by: Alberto Garcia <address@hidden>
>> ---
>>  block/qcow2-cluster.c | 56 
>> +++++++++++++++++++++++++++++++--------------------
>>  1 file changed, 34 insertions(+), 22 deletions(-)
>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>> index 57349928a9..2a53c1dc5f 100644
>> --- a/block/qcow2-cluster.c
>> +++ b/block/qcow2-cluster.c
>> @@ -264,11 +264,12 @@ int qcow2_write_l1_entry(BlockDriverState *bs, int 
>> l1_index)
>>   *
>>   */
>> -static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
>> +static int l2_allocate(BlockDriverState *bs, int l1_index)
>>  {
>>      BDRVQcow2State *s = bs->opaque;
>>      uint64_t old_l2_offset;
>> -    uint64_t *l2_table = NULL;
>> +    uint64_t *l2_slice = NULL;
>> +    unsigned slice, slice_size2, n_slices;
> I'd personally prefer size_t, but oh well.

I don't see any reason not to use int / unsigned. The size of a slice is
always <= cluster_size (an int), and both slice and n_slices are smaller
than that.

> However, I'm wondering whether this is the best approach.  The old L2
> table is probably not going to be used after this function, so we're
> basically polluting the cache here.  That was bad enough so far, but
> now that actually means wasting multiple cache entries on it.
> Sure, the code is simpler this way.  But maybe it would still be
> better to manually copy the data over from the old offset...  (As long
> as it's not much more complicated.)

You mean bypassing the cache altogether?

    qcow2_cache_flush(bs, s->l2_table_cache);
    new_table = g_malloc(s->cluster_size);
    if (old_l2_offset & L1E_OFFSET_MASK) {
        bdrv_pread(bs->file, old_l2_offset, new_table, s->cluster_size);
    } else {
        memset(new_table, 0, s->cluster_size);
    bdrv_pwrite(bs->file, new_l2_offset, new_table, s->cluster_size);



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