Re: [Qemu-devel] [PATCH 0/2] deal with BDRV_BLOCK_RAW

[Kevin Wolf <kwolf@redhat.com>]

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Am 13.08.2019 um 16:53 hat Max Reitz geschrieben:
> On 13.08.19 16:46, Vladimir Sementsov-Ogievskiy wrote:
> > 13.08.2019 17:31, Max Reitz wrote:
> >> On 13.08.19 13:51, Kevin Wolf wrote:
> >>
> >> [...]
> >>
> >>> Hm... This is a mess. :-)
> >>
> >> Just out of curiosity: Why?
> >>
> >> Aren’t there only two things we really need from the block_status
> >> infrastructure?
> >>
> >> (1) Whether something is allocated in the given layer of the backing chain,
> >>
> >> (2) Whether we know that a given range reads as zeroes.
> >>
> >> Do we really need anything else?
> >>
> > 
> > qemu-img map?
> 
> Which is a debugging tool.  So it doesn’t fall under “really” in my
> book.  If removing everything but allocation+zero information would make
> the code a lot simpler, I think that would be worth it.
> 
> > 1. We need to fix the bug somehow
> > 2. We need to fix comment about different block-status flags, as it really
> > lacks information of what actually "DATA" means (together with *file).
> > And what finally means "allocated", can you define it precisely?
> 
> As I wrote in my other mails, I think the problem is that it’s just
> unexpected that block_status automatically skips through for filters.
> It shouldn’t, that’s just black magic that the caller should not rely on.
> 
> (We see precisely here that it’s wrong, because the callers are not
> prepared for the allocation information returned to be associated with a
> different node than what they passed.)
> 
> So my definition is just “If the node has a COW backing file and
> block_status returns ‘not allocated’, the data will be there.
> Otherwise, the data is in the current node.”  Yes, that means that
> filters should appear as fully allocated.

You can do that, but then the callers need to learn to do the recursion
instead. After all, just copying everything if a filter is in the
subtree isn't the desired behaviour.

Kevin

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