Re: [Xen-devel] [PATCH] xen: handle inbound migration of VMs without ioreq server pages

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 9 Aug 2016, Paul Durrant wrote:
> > -----Original Message-----
> > From: Anthony PERARD [mailto:anthony.perard@xxxxxxxxxx]
> > Sent: 09 August 2016 16:19
> > To: Paul Durrant
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-devel@xxxxxxxxxx; Stefano
> > Stabellini
> > Subject: Re: [PATCH] xen: handle inbound migration of VMs without ioreq
> > server pages
> > 
> > On Mon, Aug 01, 2016 at 10:16:25AM +0100, Paul Durrant wrote:
> > > VMs created on older versions on Xen will not have been provisioned with
> > > pages to support creation of non-default ioreq servers. In this case
> > > the ioreq server API is not supported and QEMU's only option is to fall
> > > back to using the default ioreq server pages as it did prior to
> > > commit 3996e85c ("Xen: Use the ioreq-server API when available").
> > >
> > > This patch therefore changes the code in xen_common.h to stop
> > considering
> > > a failure of xc_hvm_create_ioreq_server() as a hard failure but simply
> > > as an indication that the guest is too old to support the ioreq server
> > > API. Instead a boolean is set to cause reversion to old behaviour such
> > > that the default ioreq server is then used.
> > >
> > > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>
> > 
> > There are just two lines too long:
> > 
> > WARNING: line over 80 characters
> > #31: FILE: include/hw/xen/xen_common.h:110:
> > +static inline int xen_get_default_ioreq_server_info(xc_interface *xc,
> > domid_t dom,
> > 
> > WARNING: line over 80 characters
> > #34: FILE: include/hw/xen/xen_common.h:113:
> > +                                                    evtchn_port_t 
> > *bufioreq_evtchn)
> > 
> > With that fixed: Acked-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> > 
> > Thanks,
> > 
> 
> Ok, I'll post v2 with those fixes and your ack. Thanks,

Thank you for fixing this bug and Thanks Anthony for the review.

I was about to commit it but my sense of style rebelled: I really don't
like all the if statements. Too many! Sorry for coming in so late in
commenting on a patch, I realize that it is unfair.

Would you be up for rewriting the fix so that instead of introducing

    if (use_default_ioreq_server) {
        return;
    }

in many functions, we turn xc_hvm_* calls into function pointers calls
that get set to the right behavior depending on the success of
xc_hvm_create_ioreq_server?


The call sites would be something like:

    ioreq_server->unmap_io_range_from_ioreq_server(xc, dom, ioservid, 0,
                                            start_addr, end_addr);

At boot time, if xc_hvm_create_ioreq_server returns error:

    ioreq_server = empty_stubs_functions;

else

    ioreq_server = useful_functions;


What do you guys think?

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