Re: [Xen-devel] [PATCH RFC v1 41/74] xen/pvh: do not mark the low 1MB as IO mem

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jan 08, 2018 at 03:30:17AM -0700, Jan Beulich wrote:
> >>> On 04.01.18 at 14:05, <wei.liu2@xxxxxxxxxx> wrote:
> > @@ -288,8 +289,12 @@ void __init arch_init_memory(void)
> >      dom_cow = domain_create(DOMID_COW, DOMCRF_dummy, 0, NULL);
> >      BUG_ON(IS_ERR(dom_cow));
> >  
> > -    /* First 1MB of RAM is historically marked as I/O. */
> > -    for ( i = 0; i < 0x100; i++ )
> > +    /*
> > +     * First 1MB of RAM is historically marked as I/O.  If we booted PVH,
> > +     * reclaim the space.  Irrespective, leave MFN 0 as special for the 
> > sake
> > +     * of 0 being a very common default value.
> > +     */
> > +    for ( i = 0; i < (pvh_boot ? 1 : 0x100); i++ )
> >          share_xen_page_with_guest(mfn_to_page(_mfn(i)),
> >                                    dom_io, XENSHARE_writable);
> 
> I can see this being valid as long as there's no firmware. What
> doesn't become clear from neither the description nor the
> comment is whether this is a necessary change, or just an
> optimization to avoid wasting these 255 pages.

It's just an optimization to not waste the low 1MiB. The shim will
work equally well without this.

Thanks, Roger.

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