Re: [Qemu-block] [Qemu-devel] [PATCH v2 0/3] block: Warn about usage of growing formats over non-growable protocols

[Paolo Bonzini]

On 07/05/2015 16:34, Kevin Wolf wrote:
> Am 07.05.2015 um 16:16 hat Paolo Bonzini geschrieben:
>>
>>
>> On 07/05/2015 16:07, Kevin Wolf wrote:
>>> This is not right for two reasons: The first is that this is
>>> BlockBackend code
>>
>> I think it would take effect for the qemu-nbd case though.
> 
> Oh, you want to change the server code rather than the client?

Yes.

> Wait... Are you saying that NBD sends a (platform specific) errno value
> over the network? :-/

Yes. :/  That said, at least the error codes that Linux places in
/usr/include/asm/errno-base.h seem to be pretty much standard---at least
Windows and most Unices share them---with the exception of EAGAIN.

I'll send a patch to NBD to standardize the set of error codes that it
sends.

> In theory, what error code the NBD server needs to send should be
> specified by the NBD protocol. Am I right to assume that it doesn't do
> that?

Nope.

> In any case, I'm not sure whether qemu's internal error code
> should change just for NBD. Producing the right error code for the
> protocol is the job of nbd_co_receive_request().

Ok, so it shouldn't reach blk_check_request at all.  But then, we should
aim at making blk_check_request's checks assertions.

>>> and it wouldn't even take effect for the qcow2 case
>>> where we're writing past EOF only on the protocol layer. The second is
>>> that -ENOSPC is only for writes and not for reads.
>>
>> This is right.
>>
>> Reads in the kernel return 0, but in QEMU we do not want that.  The code
>> currently returns -EIO, but perhaps -EINVAL is a better match.  It also
>> happens to be what Linux returns for discards.
> 
> Perhaps it is, yes. It shouldn't make a difference for guests anyway.
> (Unlike -ENOSPC for writes, which would trigger werror=enospc! That's
> most likely not what we want.)

Yes, we want the check duplicated in all BlockBackend users.  Most of
them already do it, see the work that Markus did last year I think.

Paolo