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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Dec 10, 2018 at 11:33:28AM +0100, Roger Pau Monné wrote:
> 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@xxxxxxxxxx>
> > > ---
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > Cc: andrei.semenov@xxxxxxxxx
> > > ---
> > >  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.

Fair enough.

Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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