Re: [Xen-devel] [PATCH 2/2] tools/libxl: Switch Arm guest type to PVH

[Julien Grall <julien.grall@xxxxxxx>]

Hi Roger,

Sorry for the late reply.

On 09/03/2018 03:40 PM, Roger Pau Monné wrote:
> On Mon, Sep 03, 2018 at 12:15:16PM +0100, Julien Grall wrote:
> > On 03/09/18 12:09, Julien Grall wrote:
> > > On 23/08/18 08:58, Roger Pau Monné wrote:
> > > > On Wed, Aug 22, 2018 at 06:48:05PM +0100, Julien Grall wrote:
> > > > > > > +
> > > > > > > +    b_info->type = LIBXL_DOMAIN_TYPE_PVH;
> > > > > > > +
> > > > > > > +    /*
> > > > > > > +     * They only fie ld in u.pv that matters on Arm are:
> > > > > > > kernel, cmdline,
> > > > > > > +     * ramdisk.
> > > > > > > +     */
> > > > > > > +
> > > > > > > +    if (!b_info->kernel && b_info->u.pv.kernel)
> > > > > > > +            b_info->kernel = b_info->u.pv.kernel;
> > > > > > > +
> > > > > > > +    if (!b_info->ramdisk && b_info->u.pv.ramdisk)
> > > > > > > +        b_info->ramdisk = b_info->u.pv.ramdisk;
> > > > > > > +
> > > > > > > +    if (!b_info->cmdline && b_info->u.pv.cmdline)
> > > > > > > +        b_info->cmdline = b_info->u.pv.cmdline;
> > > > > > > +
> > > > > > > +    /* Reset b_info->u.pvh to default values */
> > > > > > > +    memset(&b_info->u.pvh, 0, sizeof(b_info->u.pvh));
> > > > > > I'm afraid that's not correct. The default values for u.pvh are set
> > > > > > by libxl__domain_build_info_setdefault.
> > > > > I thought that this should be covered by the switch right after
> > > > > the call of
> > > > > libxl__arch_domain_build_info_setdefault. Did I miss anything?
> > > > Oh right, libxl__arch_domain_build_info_setdefault is called by
> > > > libxl__domain_build_info_setdefault.
> > > >
> > > > > What I wanted to do here is resetting the union to 0 so you
> > > > > don't get data
> > > > > mangled by the pv fields.
> > > > Another possible option I think would be to mark those fields as
> > > > deprecated in the IDL, and libxl__domain_build_info_copy_deprecated
> > > > will take care of copying them to the new place. In fact I think all
> > > > guest types should be using the top level kernel, ramdisk and cmdline
> > > > fields.
> > > I will have a look at it.
> > >
> > > > I'm not specially comfortable with changing the guest type in the
> > > > middle of libxl__domain_build_info_setdefault, but I also don't have a
> > > > much better suggestion apart from using the deprecation helper.
> > I forgot to answer to this bit. I don't think the deprecation helper will do
> > all the jobs. There are still PV specific parameters: slack_memkb, features,
> > e820_host.
> Those can be left inside the PV sub-struct and shouldn't be marked as
> deprecated.
>
> > Those are not necessary for Arm, if you don't zero them then you will not
> > initialize the PVH structure with default values. How do you suggest to
> > handle them?
> But I guess ARM doesn't use any of those fields (or else they should
> be moved to the pvh sub-struct anyway)?
Those fields should not be used by Arm. Looking at the current fields in 
pv, they all should be zeroed.
However, I am not sure if we can assume that will always be the case.
If we can assume it, then I think it would just be sufficient to have 
b_info->type = LIBXL_DOMAIN_TYPE_PVH in Arm.
Any thoughts?

Cheers,


> In which case allowing the user to set them in the first place seems
> like an error to me.
Agreed.

Cheers,

--
Julien Grall

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