Re: [PATCH v4.2] libxc: Defer initialization of start_page for HVM guests

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

On Fri, 2016-01-08 at 09:53 -0500, Boris Ostrovsky wrote:
> On 01/08/2016 09:30 AM, Juergen Gross wrote:
> > On 08/01/16 15:19, Boris Ostrovsky wrote:
> > > On 01/07/2016 11:57 PM, Juergen Gross wrote:
> > > > On 07/01/16 23:19, Boris Ostrovsky wrote:
> > > > > With commit 8c45adec18e0 ("libxc: create unmapped initrd in
> > > > > domain
> > > > > builder if supported") location of ramdisk may not be available
> > > > > to
> > > > > HVMlite guests by the time alloc_magic_pages_hvm() is invoked if
> > > > > the
> > > > > guest supports unmapped initrd.
> > > > > 
> > > > > So let's move ramdisk info initialization (along with a few other
> > > > > operations that are not directly related to allocating
> > > > > magic/special
> > > > > pages) from alloc_magic_pages_hvm() to bootlate_hvm().
> > > > > 
> > > > > Since we now split allocation and mapping of the start_info
> > > > > segment
> > > > > let's stash it, along with cmdline length, in xc_dom_image so
> > > > > that we
> > > > > can check whether we are mapping correctly-sized range.
> > > > > 
> > > > > We can also stop using xc_dom_image.start_info_pfn and leave it
> > > > > for
> > > > > PV(H) guests only.
> > > > > 
> > > > > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > > > > ---
> > > > > v4:
> > > > >    * See the last two paragraphs from commit message above
> > > > > 
> > > > > v4.1:
> > > > >    * Inverted testing of start_info_size in bootlate_hvm().
> > > > > 
> > > > > v4.2
> > > > >    * <facepalm> Actually do what I said I'd do in 4.1
> > > > > 
> > > > >    tools/libxc/include/xc_dom.h |    2 +
> > > > >    tools/libxc/xc_dom_x86.c     |  140
> > > > > +++++++++++++++++++++++------------------
> > > > >    2 files changed, 80 insertions(+), 62 deletions(-)
> > > > > 
> > > > > diff --git a/tools/libxc/include/xc_dom.h
> > > > > b/tools/libxc/include/xc_dom.h
> > > > > index 2460818..cac4698 100644
> > > > > --- a/tools/libxc/include/xc_dom.h
> > > > > +++ b/tools/libxc/include/xc_dom.h
> > > > > @@ -71,6 +71,7 @@ struct xc_dom_image {
> > > > >          /* arguments and parameters */
> > > > >        char *cmdline;
> > > > > +    size_t cmdline_size;
> > > > >        uint32_t f_requested[XENFEAT_NR_SUBMAPS];
> > > > >          /* info from (elf) kernel image */
> > > > > @@ -91,6 +92,7 @@ struct xc_dom_image {
> > > > >        struct xc_dom_seg p2m_seg;
> > > > >        struct xc_dom_seg pgtables_seg;
> > > > >        struct xc_dom_seg devicetree_seg;
> > > > > +    struct xc_dom_seg start_info_seg; /* HVMlite only */
> > > > Instead of adding HVM specific members here, you could make use of
> > > > dom.arch_private and use just a local structure defined in
> > > > xc_dom_x86.c.
> > > I did consider this but since we already keep type-specific segments
> > > in
> > > this structure (e.g. p2m_seg) decided to add an explicit segment for
> > > HVMlite.
> > But p2m_seg is accessed from multiple sources, while cmdline_size and
> > start_info_seg would be local to xc_dom_x86.c
> > 
> > BTW: thanks for the hint - I'll have a look whether p2m_seg can't be
> > moved to arch_private...
> > 
> > > Besides, I think to properly use it we'd need to add an arch hook and
> > > IMHO it's not worth the trouble in this case.
> > Why would you need another arch hook? Just add the arch_private_size
> > member to struct xc_dom_arch and everything is set up for you. Look
> > how it is handled for the pv case in xc_dom_x86.c
> 
> So it is already hooked up, I didn't notice that we do register 
> xc_hvm_32, even though arch_private_size is 0.
> 
> This would be a type-specific area though, not arch-specific as the name 
> implies. So perhaps xc_dom_image_x86 should be modified to include 
> type-specific structures (via a union).

You are talking future work here, right? There's no reason not to proceed
with the current patch AFAICT, I'm really just giving Roger a chance to
have a look at this point.

BTW, it might be possible to use xc_dom_seg_to_ptr instead of an open coded
xc_map_foreign? Although it wasn't used before and maybe there is a reason
for that which still applies.

Ian.

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