Re: [PATCH v2 3/5] libxc: create unmapped initrd in domain builder if supported

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-10-02 at 07:49 +0200, Juergen Gross wrote:
> In case the kernel of a new pv-domU indicates it is supporting an
> unmapped initrd, don't waste precious virtual space for the initrd,
> but allocate only guest physical memory for it.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  tools/libxc/xc_dom_core.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/libxc/xc_dom_core.c b/tools/libxc/xc_dom_core.c
> index b510bbd..85b531a 100644
> --- a/tools/libxc/xc_dom_core.c
> +++ b/tools/libxc/xc_dom_core.c
> @@ -1019,8 +1019,9 @@ int xc_dom_build_image(struct xc_dom_image *dom)
>      if ( dom->kernel_loader->loader(dom) != 0 )
>          goto err;
>  
> -    /* load ramdisk */
> -    if ( dom->ramdisk_blob )
> +    /* Load ramdisk if initial mapping required. */
> +    if ( dom->ramdisk_blob &&
> +         (!dom->parms.mod_start_pfn || dom->ramdisk_seg.vstart) )
>      {
>          if ( xc_dom_build_ramdisk(dom) != 0 )
>              goto err;
> @@ -1063,6 +1064,23 @@ int xc_dom_build_image(struct xc_dom_image *dom)
>                __FUNCTION__, dom->virt_alloc_end);
>      DOMPRINTF("%-20s: virt_pgtab_end : 0x%" PRIx64 "",
>                __FUNCTION__, dom->virt_pgtab_end);
> +
> +    /* Prepare allocating unmapped memory. */
> +    if ( dom->virt_pgtab_end )
> +        dom->virt_alloc_end = dom->virt_pgtab_end;
> +
> +    /* Load ramdisk if no initial mapping required. */
> +    if ( dom->ramdisk_blob && !dom->ramdisk_seg.vstart &&
> +         dom->parms.mod_start_pfn )
> +    {
> +        if ( xc_dom_build_ramdisk(dom) != 0 )
> +            goto err;
> +        dom->flags |= SIF_MOD_START_PFN;
> +        dom->ramdisk_seg.vend = dom->ramdisk_seg.vend - dom->ramdisk_seg.vstart;
> +        dom->ramdisk_seg.vstart = dom->ramdisk_seg.pfn;
> +        dom->ramdisk_seg.vend += dom->ramdisk_seg.vstart;

This seems like it is trying to do something clever, like partially
reversing something which the xc_dom_alloc_segment call in
 xc_dom_build_ramdisk has done.

It looks like the vend handling in particular is just a complicated way of
subtracting vstart and adding pfn, with the aim of rebasing from virt to
phys world.

Plus vstart/end are addresses, while presumably pfn is a page number, so
I'm confused about that as well.

I'm also not clear how/where the virtual mapping is avoided, given that
this code here does strictly more than the original code above which is now
made conditional does.

Ian.