Re: [Qemu-devel] [PATCH 1/3] block/qcow2: refactoring of threaded encryption code

[Eric Blake]

On 9/6/19 1:55 PM, Maxim Levitsky wrote:

>>> +/*
>>> + * qcow2_co_encrypt()
>>> + *
>>> + * Encrypts a sector size aligned contiguous area
>>> + *
>>> + * @host_cluster_offset - on disk offset of the cluster in which
>>> + *                        the buffer resides
>>> + *
>>> + * @guest_offset - guest (virtual) offset of the buffer
>>> + * @buf - buffer with the data to encrypt
>>> + * @len - length of the buffer
>>> + *
>>> + * Note that the area is not cluster aligned and might cross a cluster
>>> + * boundary
>>
>> Umm, how is it possible for a sector to cross a cluster boundary?  All
>> clusters are sector-aligned, and encryption only works on aligned
>> sectors.  Oh, I see - if @len is a multiple larger than sector size,
>> then we have multiple sectors, and then indeed we may cross clusters.
>> But then the docs about being 'a sector size aligned contiguous area' is
>> not quite right.
> 
> Why? the written area is always both aligned on _sector_ boundary
> and multiple of the sector size. At least that what I see from
> the existing asserts.

I'm thinking it should read something like:

Encrypts one or more contiguous aligned sectors

to make it obvious that because there can be multiple sectors, there may
indeed be a cluster boundary mid-length.  My complaint was that the
original text made it sound like there was exactly one sector (at which
point @len is redundant, unless sectors are variably-sized)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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