Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs

[Max Reitz]

On 03.04.2017 10:15, Kevin Wolf wrote:
> Am 31.03.2017 um 19:43 hat Max Reitz geschrieben:

[...]

>> So in theory all that's necessary is to set share-rw=on for the device
>> in the management layer. But I'm not sure whether that's practical.
> 
> Yes, libvirt needs to provide this option if the guest supports sharing.
> If it doesn't support sharing, rejecting a read-write NBD client seems
> correct to me.
> 
> Peter, Eric, what is the status on the libvirt side here?
> 
>> As for just allowing the NBD server write access to the device... To me
>> that appears pretty difficult from an implementation perspective. We
>> assert that nobody can write without having requested write access and
>> we make sure that nobody can request write access without it being
>> allowed. Making an exception for NBD seems very difficult and would
>> probably mean we'd have to drop the assertion for write accesses altogether.
> 
> Making an exception would simply be wrong.

Indeed. That is why it would be so difficult.

The question remains whether it is practical not to make an exception.
As far as I know, libvirt is only guaranteed to support older qemu
versions, not necessarily future ones. So we should be allowed to break
existing use cases here until libvirt is updated (assuming it is
possible for libvirt to express "guest device allows shared writes" as
an option for its next release).

Max

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