Re: [PATCH v2 2/2] x86/dom0: improve paging memory usage calculations

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Dec 06, 2018 at 12:42:15PM +0000, Wei Liu wrote:
> On Wed, Dec 05, 2018 at 03:55:00PM +0100, Roger Pau Monne wrote:
> > Current approximation of paging memory usage is based on the required
> > amount when running in shadow mode and doesn't take into account the
> > memory required by the IOMMU page tables.
> > 
> > Fix this by introducing a function to calculate the amount of memory
> > required by HAP/IOMMU page tables. The formula used to calculate such
> > approximation is based on the pessimistic approach that each 4KB
> > memory chunk will use 8 bytes of page table memory. Note that this
> > approximation might need further tuning based on testing on different
> > systems.
> > 
> > Also fix the calculation of the required paging related memory in
> > dom0_compute_nr_pages to take into account the paging implementation
> > (shadow or HAP) and whether the IOMMU pages tables are shared with the
> > HAP page tables.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> > ---
> > Cc: Jan Beulich <jbeulich@suse.com>
> > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> > Cc: Wei Liu <wei.liu2@citrix.com>
> > Cc: andrei.semenov@bertin.fr
> > ---
> >  xen/arch/x86/dom0_build.c        | 31 +++++++++++++++++++++++++++----
> >  xen/arch/x86/hvm/dom0_build.c    |  6 ++++--
> >  xen/include/asm-x86/dom0_build.h |  2 ++
> >  3 files changed, 33 insertions(+), 6 deletions(-)
> > 
> > diff --git a/xen/arch/x86/dom0_build.c b/xen/arch/x86/dom0_build.c
> > index ba9aa85611..3a8e138f23 100644
> > --- a/xen/arch/x86/dom0_build.c
> > +++ b/xen/arch/x86/dom0_build.c
> > @@ -267,6 +267,25 @@ unsigned long __init dom0_shadow_pages(const struct domain *d,
> >      return ((memkb + 1023) / 1024) << (20 - PAGE_SHIFT);
> >  }
> >  
> > +unsigned long __init dom0_hap_pages(const struct domain *d,
> > +                                    unsigned long nr_pages)
> > +{
> > +    /*
> > +     * Attempt to account for at least some of the MMIO regions by adding the
> > +     * size of the holes in the memory map to the amount of pages to map. Note
> > +     * this will obviously not account for MMIO regions that are past the last
> > +     * RAM range in the memory map.
> > +     */
> > +    nr_pages += max_page - total_pages;
> 
> Do those regions past end of RAM range show up in E820 map?

No, BARs for example don't need to be in reserved regions. I've got
one box with a 16GB Tesla card that has the 16GB BAR placed way past
the last entry in the memory map, without any reserved region.

So while this approach is not perfect, it's better than what we
currently do, and we can always improve from there if it's clear what
limitations we currently have.

Thanks, Roger.

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