Re: [Xen-devel] [PATCH] linux-2.6.18/x86_64: remove an inapplicable feature flag

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2010-11-03 at 14:04 +0000, Jan Beulich wrote:
> >>> On 03.11.10 at 13:35, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> > On 11/03/2010 05:13 AM, Jan Beulich wrote:
> >>>>> On 03.11.10 at 09:59, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> >>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
> >>>
> >>> --- a/arch/x86_64/kernel/head-xen.S
> >>> +++ b/arch/x86_64/kernel/head-xen.S
> >>> @@ -206,6 +206,6 @@
> >>>   ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          .quad,  startup_64)
> >>>   ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .quad,  hypercall_page)
> >>>   ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID,   .quad,  
> >>> _PAGE_PRESENT,_PAGE_PRESENT)
> >>> - ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .asciz, 
> > "writable_page_tables|writable_descriptor_tables|auto_translated_physmap|pae_
> > pgdir_above_4gb|supervisor_mode_kernel")
> >>> + ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .asciz, 
> > "writable_page_tables|writable_descriptor_tables|auto_translated_physmap|supe
> > rvisor_mode_kernel")
> >>>   ELFNOTE(Xen, XEN_ELFNOTE_LOADER,         .asciz, "generic")
> >>>   ELFNOTE(Xen, XEN_ELFNOTE_SUSPEND_CANCEL, .long,  1)
> >> If the flag was removed from the pv-ops 64-bit kernel too, we could
> >> even start warning about inapplicable flags in hypervisor/tools.
> >>
> >> What puzzles me more though is that the pv-ops kernel requires
> >> the writable_page_tables feature. Why is that?
> > 
> > It did in the very early days, but it hasn't used it since converting to
> > PV MMU (couldn't, since it doesn't exist any more).  It lists
> > "!writable_page_table", which I read as meaning "writable page tables
> > not supported"; does it actually mean "must"?
> 
> Yes, it does (according to xen/include/public/elfnote.h).

The selection of ! for this meaning was very ill-conceived! I don't know
what I was thinking...

I think it is correct to remove this flag. The pvops kernel has no
support for running in a mode where page tables are not read-only, which
is not to be confused with the trap-n-emulate type of writeable page
table update which it does use.

Ian.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel