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Re: [PATCH v5 6/6] multifd: Implement zerocopy write in multifd migratio


From: Daniel P . Berrangé
Subject: Re: [PATCH v5 6/6] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy)
Date: Tue, 16 Nov 2021 16:39:53 +0000
User-agent: Mutt/2.0.7 (2021-05-04)

On Tue, Nov 16, 2021 at 05:34:50PM +0100, Juan Quintela wrote:
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> >> 
> >>     if (params->zerocopy &&
> >>         (params->parameters.multifd_compression != 
> >> MULTIFD_COMPRESSION_NONE ||
> >>          migrate_use_tls())) {
> >>            error_setg(&err,
> >>                      "Zerocopy only available for non-compressed non-TLS 
> >> multifd migration");
> >>         return false;
> >>     }
> >> 
> >> You have to do the equivalent of multifd_compression and tls enablement,
> >> to see that zerocopy is not enabled, of course.
> >> 
> >> I would prefer to check for QIO_CHANNEL_FEATUR_WRITE_ZEROCPY there, but
> >> I can't see a way of doing that without a qio.
> >
> > I don't think you need to check that feature flag.
> 
> Oh, I mean other thing.
> 
> When you set "zerocopy" capability, you don't know if the kernel support
> it.  My understanding is that the only way to check if it supported is
> here.

If you reqest it and it isn't supported you'll get an error back from
qio_channel_writev_zerocopy(). That's a bit too late though.

Ideally we should report an error straight after the migration code
creates the I/O channel, by querying for the feature.


Regards,
Daniel
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