Re: [Xen-devel] [PATCH Remus v5 1/2] libxc/save: implement Remus checkpointed save

[Yang Hongyang <yanghy@xxxxxxxxxxxxxx>]

On 05/15/2015 05:31 PM, Andrew Cooper wrote:
> On 15/05/15 09:37, Yang Hongyang wrote:
> > On 05/15/2015 04:29 PM, Andrew Cooper wrote:
> > > On 15/05/2015 03:12, Yang Hongyang wrote:
> > > > > > @@ -467,6 +477,14 @@ static int send_domain_memory_live(struct
> > > > > > xc_sr_context *ctx)
> > > > > >         DECLARE_HYPERCALL_BUFFER_SHADOW(unsigned long, dirty_bitmap,
> > > > > >                                         &ctx->save.dirty_bitmap_hbuf);
> > > > > >
> > > > > > +    /*
> > > > > > +     * With Remus, we will enter checkpointed save after live
> > > > > > migration.
> > > > > > +     * In checkpointed save loop, we skip the live part and pause
> > > > > > straight
> > > > > > +     * away to send dirty pages between checkpoints.
> > > > > > +     */
> > > > > > +    if ( !ctx->save.live )
> > > > > > +        goto last_iter;
> > > > > Rather than use goto would it work to refactor everything from here to
> > > > > the label into some sort of helper and just call that in the "actually
> > > > > live" case?
> > > > >
> > > > > Or perhaps everything from the label to the end should be a helper
> > > > > function which the caller can also use in thecheckpoint case
> > > > > instead of
> > > > > calling send_domain_memory_live (and which s_d_m_l also calls of
> > > > > course).
> > > > I'm going to refactor the send_domain_memory_live() as follows:
> > > >
> > > > split the send_domain_memory_live() into three helper function:
> > > >     - send_memory_live()  do the actually live case
> > > >     - suspend_and_send_dirty() suspend the guest and send dirty pages
> > > >     - send_memory_verify()
> > > >
> > > > then:
> > > >     - send_domain_memory_live() combination of those three helper
> > > > functions
> > > >     - send_domain_momory_checkpointed() calls
> > > > suspend_and_send_dirty() and
> > > >                                         send_memory_verify()
> > > >     - send_domain_memory_nonlive() stay as it is
> > > > Does it make sense?
> > > verify mode is only a debugging tool, and shouldn't be used in general.
> > > (It is actually a lot less useful in retrospect than one would imagine.)
> > Do you mean we do not need this in checkpointed stream?
> > Currently, tt is conditioned under 'if ( ctx->save.debug )', both in
> > live migration and checkpointed stream.
> Ah - you absolutely don't want it in a checkpointed stream.  As soon as
> that option has been set, the restoring side can no longer resume the
> domain.
So it needs to be conditioned under
if ( ctx->save.debug && !ctx->save.checkpointed )

if it is a checkpointed stream, we just ignore this flag, is it OK?

> Basically, it tells the restore side to start doing memcmp() instead of
> memcpy() on the incoming page data.  The initial hope was to detect
> memory corruption, but it turns out that there is natural memory
> corruption to be had anyway when PV drivers are in use.
>
> (This is another area which is around for compatibility with legacy,
> rather than actually of being much use in practice)
>
> ~Andrew
> .
--
Thanks,
Yang.

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