Re: [Qemu-devel] [PATCH 2/5] migration: add the interface to set get_return_path

[858585 jemmy]

On Thu, Apr 12, 2018 at 4:28 PM, Daniel P. Berrangé <address@hidden> wrote:
> On Wed, Apr 11, 2018 at 06:18:18PM +0100, Dr. David Alan Gilbert wrote:
>> * Lidong Chen (address@hidden) wrote:
>> > The default get_return_path function of iochannel does not work for
>> > RDMA live migration. So add the interface to set get_return_path.
>> >
>> > Signed-off-by: Lidong Chen <address@hidden>
>>
>> Lets see how Dan wants this done, he knows the channel/file stuff;
>> to me this feels like it should be adding a member to QIOChannelClass
>> that gets used by QEMUFile's get_return_path.
>
> No that doesn't really fit the model. IMHO the entire concept of a separate
> return path object is really wrong. The QIOChannel implementations are
> (almost) all capable of bi-directional I/O, which is why the the 
> get_retun_path
> function just creates a second QEMUFile pointing to the same QIOChannel
> object we already had. Migration only needs the second QEMUFile, because that
> struct re-uses the same struct fields for tracking different bits of info
> depending on which direction you're doing I/O in. A real fix would be to
> stop overloading the same fields for multiple purposes in the QEMUFile, so
> that we only needed a single QEMUFile instance.
>
> Ignoring that though, the particular problem we're facing here is that the
> QIOChannelRDMA impl that is used is not written in a way that allows
> bi-directional I/O, despite the RDMA code it uses being capable of it.
>
> So rather than changing this get_return_path code, IMHO, the right fix to
> simply improve the QIOChannelRDMA impl so that it fully supports 
> bi-directional
> I/O like all the other channels do.

Hi Daniel:
     Thanks for your suggestion.
     I will have a try.

>
> Regards,
> Daniel
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