Re: [Xen-devel] [PATCH v1 0/8]: PVH (PV guest with extensions)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2012-10-04 at 13:52 +0100, Konrad Rzeszutek Wilk wrote:
> On Thu, Oct 4, 2012 at 4:15 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Wed, 2012-10-03 at 21:39 +0100, Konrad Rzeszutek Wilk wrote:
> >> > You Ok with XEN_ELFNOTE_GUEST_PVH, or should I call it
> >> > XEN_ELFNOTE_PV_EXTENSIONS. Please LMK.
> >
> > My intention was that you add XENFEAT_supervisor_mode_kernel et al. to
> > the existing XEN_ELFNOTE_FEATURES note rather than adding a whole new
> > note. I some how missed that this is what you were doing above. e.g.
> > what I meant was:
> >
> > in Kconfig:
> > config XEN_X86_PVH
> >         bool "Support for running as a PVH guest (EXPERIMENTAL)"
> >         depends X86_64 && <..etc..> && EXPERIMENTAL
> >         help
> >                         This option enables support for running as a PVH
> >                         guest (PV guest using hardware extensions) under
> >                         a suitably capable hypervisor.
> >
> >                         This option is EXPERIMETNAL because the
> >                         hypervisor interfaces which it uses are not yet
> >                         considered stable therefore backwards and
> >                         forwards compatibility is not yet guaranteed.
> >
> >                         If unsure, say N.
> >
> > Adjust the depends to suit reality.
> >
> > I thought we had that second paragraph for ARM too, but it seems like I
> > was wrong, we should probably add it.
> >
> > then in xen-head.S (adjusted for features actually used by PVH):
> > #ifdef CONFIG_XEN_X86_PVH
> > #define FEATURES_PVH "XENFEAT_writable_descriptor_tables|" \
> >                      "XENFEAT_auto_translated_physmap|" \
> >                      "XENFEAT_supervisor_mode_kernel" \
> >                      "XENFEAT_hvm_callback_vector"
> > #else
> > #define FEATURES_PVH /* Not supported */
> > #endif
> > ...
> > ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .asciz 
> > "!writable_page_tables|pae_pgdir_above_4gb"FEATURES_PVH)
> 
> As long as those new fields exists with the older hypervisors - that
> is OK. If you define new
> ones the old hypervisors (say Xen 4.1) will refuse booting the kernel.

The note declares features which can be supported by this kernel but
they are not mandatory unless you prefix them with a "!" so assuming you
don't put a ! in an older hypervisor will boot the kernel just fine. It
just won't enable PVH, which is what we want.

Ian.


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