Re: [Xen-devel] [PATCH] xen/balloon: do not modify the p2m for auto_translate guests

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Dec 13, 2013 at 02:47:45PM +0000, Stefano Stabellini wrote:
> On Fri, 13 Dec 2013, Konrad Rzeszutek Wilk wrote:
> > On Wed, Dec 11, 2013 at 04:58:42PM +0000, Stefano Stabellini wrote:
> > > decrease_reservation doesn't modify the p2m for auto_translate guests,
> > > but increase_reservation does.
> > > Fix that by avoiding any p2m modifications in both increase_reservation
> > > and decrease_reservation for auto_translated guests.
> > > 
> > > Avoid allocating or using scratch pages for auto_translated guests.
> > > 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/xen/balloon.c |   63 
> > > ++++++++++++++++++++++++++-----------------------
> > >  1 file changed, 34 insertions(+), 29 deletions(-)
> > > 
> > > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > > index 55ea73f..4c02e2b 100644
> > > --- a/drivers/xen/balloon.c
> > > +++ b/drivers/xen/balloon.c
> > > @@ -350,17 +350,19 @@ static enum bp_state increase_reservation(unsigned 
> > > long nr_pages)
> > >  
> > >           pfn = page_to_pfn(page);
> > >  
> > > -         set_phys_to_machine(pfn, frame_list[i]);
> > > -
> > >  #ifdef CONFIG_XEN_HAVE_PVMMU
> > > -         /* Link back into the page tables if not highmem. */
> > > -         if (xen_pv_domain() && !PageHighMem(page)) {
> > > -                 int ret;
> > > -                 ret = HYPERVISOR_update_va_mapping(
> > > -                         (unsigned long)__va(pfn << PAGE_SHIFT),
> > > -                         mfn_pte(frame_list[i], PAGE_KERNEL),
> > > -                         0);
> > > -                 BUG_ON(ret);
> > > +         if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> > > +                 set_phys_to_machine(pfn, frame_list[i]);
> > > +
> > > +                 /* Link back into the page tables if not highmem. */
> > > +                 if (!PageHighMem(page)) {
> > > +                         int ret;
> > > +                         ret = HYPERVISOR_update_va_mapping(
> > > +                                         (unsigned long)__va(pfn << 
> > > PAGE_SHIFT),
> > > +                                         mfn_pte(frame_list[i], 
> > > PAGE_KERNEL),
> > > +                                         0);
> > > +                         BUG_ON(ret);
> > > +                 }
> > >           }
> > >  #endif
> > >  
> > > @@ -378,7 +380,6 @@ static enum bp_state decrease_reservation(unsigned 
> > > long nr_pages, gfp_t gfp)
> > >   enum bp_state state = BP_DONE;
> > >   unsigned long  pfn, i;
> > >   struct page   *page;
> > > - struct page   *scratch_page;
> > >   int ret;
> > >   struct xen_memory_reservation reservation = {
> > >           .address_bits = 0,
> > > @@ -411,27 +412,29 @@ static enum bp_state decrease_reservation(unsigned 
> > > long nr_pages, gfp_t gfp)
> > >  
> > >           scrub_page(page);
> > >  
> > > +#ifdef CONFIG_XEN_HAVE_PVMMU
> > >           /*
> > >            * Ballooned out frames are effectively replaced with
> > >            * a scratch frame.  Ensure direct mappings and the
> > >            * p2m are consistent.
> > >            */
> > > -         scratch_page = get_balloon_scratch_page();
> > > -#ifdef CONFIG_XEN_HAVE_PVMMU
> > > -         if (xen_pv_domain() && !PageHighMem(page)) {
> > > -                 ret = HYPERVISOR_update_va_mapping(
> > > -                         (unsigned long)__va(pfn << PAGE_SHIFT),
> > > -                         pfn_pte(page_to_pfn(scratch_page),
> > > -                                 PAGE_KERNEL_RO), 0);
> > > -                 BUG_ON(ret);
> > > -         }
> > > -#endif
> > >           if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> > >                   unsigned long p;
> > > +                 struct page   *scratch_page = 
> > > get_balloon_scratch_page();
> > > +
> > > +                 if (!PageHighMem(page)) {
> > 
> > How come you removed the 'xen_pv_domain' part?
> 
> I thought that it's meaningless here. What we care about is whether it
> is an auto_translated guests or not, everything else is irrelevant.
> It also goes in the direction of using specific tests for things instead
> of catch all tests like xen_pv_domain that don't mean much anymore.

Please mention that in the commit description. Actually looking at
the description you could say:

"xen/balloon: Seperate the auto-translate logic properly.

The auto-xlat logic vs the non-xlat means that we don't need to for
auto-xlat guests (like PVH, HVM or ARM):
 - use P2M
 - use scratch page.

However the code in increase_reservation does modify the p2m for
auto_translate guests, but not in decrease_reservation.

Fix that by avoiding any p2m modifications in both increase_reservation
and decrease_reservation for auto_translated guests.

And also avoid allocating or using scratch pages for auto_translated guests.

Lastly, since !auto-xlat is really another way of saying 'xen_pv'
remove the redundant 'xen_pv_domain' check.
"

or so.

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